[PATCH v5 0/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[PATCH v5 0/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[Hao Ge]

v3 was a single patch. After discussion with Suren and Andrew we went
for a more graceful approach: rather than failing the module load on
overflow, let it load without profiling. Once profiling is disabled,
codetag_needs_module_section() returns false, so on retry the codetag
section is placed as regular module data.

A new patch (1/2) is added to move release_module_tags() above
reserve_module_tags(), since the overflow path now has to call it and
the helper sits below it.

release_module_tags() is what module unload calls to drop a module's
reservation from the maple tree. By the time reserve_module_tags()
detects the overflow it has already stored that reservation, and the
-EAGAIN return skips vm_module_tags_populate(), so the backing pages
never get mapped. If reserve_module_tags() returns without calling
release_module_tags(), the stale entry keeps pointing at that unmapped
range; when the module is later unloaded, release_module_tags() walks
it and panics.

Tested on an x86_64 virtual machine:

# insmod overflow_tag.ko
# dmesg
  With module overflow_tag there are too many tags to fit in 13 page
  flag bits. Memory allocation profiling is disabled!
# rmmod overflow_tag
The module loads without profiling.

Changes in v5:
- add Fixes: and Cc: stable to patch 1/2 as well, since 2/2 does not
  compile without it (Andrew Morton)
- restore frob-adjusted mem[type].size on retry instead of zeroing,
  as s390 and parisc add GOT/PLT space there in
  module_frob_arch_sections() (Reported by Sashiko)
- drop the load_module() mem_profiling_support check; the percpu
  counter leak is pre-existing and orthogonal to this fix

Changes in v4:
- add a new patch (1/2) to move release_module_tags() above
  reserve_module_tags(); the overflow fix is 2/2
- release the reservation on the -EAGAIN path
- return -EAGAIN instead of -ENOMEM so the module can still load
  without profiling (Suren)
- reset sh_addr, mem[type].size and sym/str SHF_ALLOC before retry
- skip percpu counters in load_module() when profiling is off

Changes in v3:
- use pr_warn_once() instead of pr_warn()
- return -ENOMEM instead of -ENOSPC (Suren)
- expand the commit message to describe the /proc/allocinfo impact
  (Andrew)

Changes in v2:
- return an error after shutdown_mem_profiling() to skip
  vm_module_tags_populate()

v1: https://lore.kernel.org/all/20260804064408.105033-1-hao.ge@linux.dev/
v2: https://lore.kernel.org/all/20260804122038.190270-1-hao.ge@linux.dev/
v3: https://lore.kernel.org/all/20260805090633.141001-1-hao.ge@linux.dev/
v4: https://lore.kernel.org/all/20260810093955.153015-1-hao.ge@linux.dev/

Hao Ge (2):
  alloc_tag: move release_module_tags() above reserve_module_tags()
  alloc_tag: fix undetected compressed tag overflow when profiling is
    disabled

 kernel/module/main.c |  25 ++++++++++-
 mm/alloc_tag.c       | 100 ++++++++++++++++++++++---------------------
 2 files changed, 74 insertions(+), 51 deletions(-)

--
2.25.1

[PATCH v5 1/2] alloc_tag: move release_module_tags() above reserve_module_tags()

[Hao Ge]

release_module_tags() is a cleanup helper. reserve_module_tags() can
also fail after storing the reservation in the maple tree, in which
case it should call release_module_tags() to undo it. Move the helper
above reserve_module_tags() so no forward declaration is needed.

No functional change.

Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
Cc: stable@vger.kernel.org
Signed-off-by: Hao Ge <hao.ge@linux.dev>
---
 mm/alloc_tag.c | 92 +++++++++++++++++++++++++-------------------------
 1 file changed, 46 insertions(+), 46 deletions(-)

diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
index 52aece27b00e..af44f90379f2 100644
--- a/mm/alloc_tag.c
+++ b/mm/alloc_tag.c
@@ -835,6 +835,52 @@ static int vm_module_tags_populate(void)
 	return 0;
 }
 
+static void release_module_tags(struct module *mod, bool used)
+{
+	MA_STATE(mas, &mod_area_mt, module_tags.size, module_tags.size);
+	struct alloc_tag *start_tag;
+	struct alloc_tag *end_tag;
+	struct module *val;
+
+	mas_lock(&mas);
+	mas_for_each_rev(&mas, val, 0)
+		if (val == mod)
+			break;
+
+	if (!val) /* module not found */
+		goto out;
+
+	if (!used)
+		goto release_area;
+
+	start_tag = (struct alloc_tag *)(module_tags.start_addr + mas.index);
+	end_tag = (struct alloc_tag *)(module_tags.start_addr + mas.last);
+	if (!clean_unused_counters(start_tag, end_tag)) {
+		struct alloc_tag *tag;
+
+		for (tag = start_tag; tag <= end_tag; tag++) {
+			struct alloc_tag_counters counter;
+
+			if (!tag->counters)
+				continue;
+
+			counter = alloc_tag_read(tag);
+			pr_info("%s:%u module %s func:%s has %llu allocated at module unload\n",
+				tag->ct.filename, tag->ct.lineno, tag->ct.modname,
+				tag->ct.function, counter.bytes);
+		}
+	} else {
+		used = false;
+	}
+release_area:
+	mas_store(&mas, used ? &unloaded_mod : NULL);
+	val = mas_prev_range(&mas, 0);
+	if (val == &prepend_mod)
+		mas_store(&mas, NULL);
+out:
+	mas_unlock(&mas);
+}
+
 static void *reserve_module_tags(struct module *mod, unsigned long size,
 				 unsigned int prepend, unsigned long align)
 {
@@ -922,52 +968,6 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
 	return (struct alloc_tag *)(module_tags.start_addr + offset);
 }
 
-static void release_module_tags(struct module *mod, bool used)
-{
-	MA_STATE(mas, &mod_area_mt, module_tags.size, module_tags.size);
-	struct alloc_tag *start_tag;
-	struct alloc_tag *end_tag;
-	struct module *val;
-
-	mas_lock(&mas);
-	mas_for_each_rev(&mas, val, 0)
-		if (val == mod)
-			break;
-
-	if (!val) /* module not found */
-		goto out;
-
-	if (!used)
-		goto release_area;
-
-	start_tag = (struct alloc_tag *)(module_tags.start_addr + mas.index);
-	end_tag = (struct alloc_tag *)(module_tags.start_addr + mas.last);
-	if (!clean_unused_counters(start_tag, end_tag)) {
-		struct alloc_tag *tag;
-
-		for (tag = start_tag; tag <= end_tag; tag++) {
-			struct alloc_tag_counters counter;
-
-			if (!tag->counters)
-				continue;
-
-			counter = alloc_tag_read(tag);
-			pr_info("%s:%u module %s func:%s has %llu allocated at module unload\n",
-				tag->ct.filename, tag->ct.lineno, tag->ct.modname,
-				tag->ct.function, counter.bytes);
-		}
-	} else {
-		used = false;
-	}
-release_area:
-	mas_store(&mas, used ? &unloaded_mod : NULL);
-	val = mas_prev_range(&mas, 0);
-	if (val == &prepend_mod)
-		mas_store(&mas, NULL);
-out:
-	mas_unlock(&mas);
-}
-
 static int load_module(struct module *mod, struct codetag *start, struct codetag *stop)
 {
 	/* Allocate module alloc_tag percpu counters */
-- 
2.25.1

[PATCH v5 2/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[Hao Ge]

In reserve_module_tags(), the tag overflow check is gated on
mem_alloc_profiling_enabled():

    if (mem_alloc_profiling_enabled() && !tags_addressable())

If profiling is toggled off at runtime and a module is loaded whose
tags exceed the compressed-mode limit, shutdown_mem_profiling() is
skipped. vm_module_tags_populate() still maps memory for the tags and
the module loads successfully, but the total tag count now exceeds what
NR_UNUSED_PAGEFLAG_BITS can address.

Once profiling is re-enabled, ref_to_idx() computes each tag's index
as its position in the alloc_tag array. update_page_tag_ref() masks
it to alloc_tag_ref_mask before storing in page->flags. Indices
beyond the mask are truncated and idx_to_ref() resolves them to wrong
tags.

This silently corrupts /proc/allocinfo: allocated pages get attributed
to the wrong call sites, so the statistics it reports are wrong.

mem_alloc_profiling_enabled() and mem_profiling_compressed are
independent. Once compressed mode is established at boot, it stays
active regardless of runtime toggles of mem_profiling.

Remove the mem_alloc_profiling_enabled() guard. On overflow, shut down
profiling, release the reservation, and return -EAGAIN so that
layout_and_allocate() retries with profiling disabled: codetag sections
are then placed as regular module data and the module loads without
profiling rather than being rejected entirely.

Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
Cc: stable@vger.kernel.org
Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <hao.ge@linux.dev>
---
 kernel/module/main.c | 25 +++++++++++++++++++++++--
 mm/alloc_tag.c       |  8 +++++---
 2 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/kernel/module/main.c b/kernel/module/main.c
index 46dd8d25a605..ed26f167be84 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -2944,6 +2944,7 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
 {
 	struct module *mod;
 	int err;
+	unsigned long frob_size[MOD_MEM_NUM_TYPES];
 
 	/* Allow arches to frob section contents and sizes.  */
 	err = module_frob_arch_sections(info->hdr, info->sechdrs,
@@ -2966,18 +2967,38 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
 	 */
 	module_mark_ro_after_init(info->hdr, info->sechdrs, info->secstrings);
 
+	/*
+	 * Save the sizes reserved by module_frob_arch_sections() so they can
+	 * be restored if we retry below.
+	 */
+	for_each_mod_mem_type(type)
+		frob_size[type] = info->mod->mem[type].size;
+
 	/*
 	 * Determine total sizes, and put offsets in sh_entsize.  For now
 	 * this is done generically; there doesn't appear to be any
 	 * special cases for the architectures.
 	 */
+retry:
 	layout_sections(info->mod, info);
 	layout_symtab(info->mod, info);
 
 	/* Allocate and move to the final place */
 	err = move_module(info->mod, info);
-	if (err)
-		return ERR_PTR(err);
+	if (err) {
+		if (err != -EAGAIN)
+			return ERR_PTR(err);
+		/*
+		 * -EAGAIN means profiling was disabled but the module
+		 * can still load without it. Reset state and retry.
+		 */
+		rewrite_section_headers(info, flags);
+		for_each_mod_mem_type(type)
+			info->mod->mem[type].size = frob_size[type];
+		info->sechdrs[info->index.sym].sh_flags &= ~(unsigned long)SHF_ALLOC;
+		info->sechdrs[info->index.str].sh_flags &= ~(unsigned long)SHF_ALLOC;
+		goto retry;
+	}
 
 	/* Module has been copied to its final place now: return it. */
 	mod = (void *)info->sechdrs[info->index.mod].sh_addr;
diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
index af44f90379f2..0a7b657fe2de 100644
--- a/mm/alloc_tag.c
+++ b/mm/alloc_tag.c
@@ -950,10 +950,12 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
 		int grow_res;
 
 		module_tags.size = offset + size;
-		if (mem_alloc_profiling_enabled() && !tags_addressable()) {
+		if (!tags_addressable()) {
 			shutdown_mem_profiling(true);
-			pr_warn("With module %s there are too many tags to fit in %d page flag bits. Memory allocation profiling is disabled!\n",
-				mod->name, NR_UNUSED_PAGEFLAG_BITS);
+			pr_warn_once("With module %s there are too many tags to fit in %d page flag bits. Memory allocation profiling is disabled!\n",
+				     mod->name, NR_UNUSED_PAGEFLAG_BITS);
+			release_module_tags(mod, false);
+			return ERR_PTR(-EAGAIN);
 		}
 
 		grow_res = vm_module_tags_populate();
-- 
2.25.1

Re: [PATCH v5 1/2] alloc_tag: move release_module_tags() above reserve_module_tags()

[Suren Baghdasaryan]

On Tue, Aug 11, 2026 at 10:41 PM Hao Ge <hao.ge@linux.dev> wrote:
>
> release_module_tags() is a cleanup helper. reserve_module_tags() can
> also fail after storing the reservation in the maple tree, in which
> case it should call release_module_tags() to undo it. Move the helper
> above reserve_module_tags() so no forward declaration is needed.
>
> No functional change.
>
> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> Cc: stable@vger.kernel.org
> Signed-off-by: Hao Ge <hao.ge@linux.dev>

Acked-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  mm/alloc_tag.c | 92 +++++++++++++++++++++++++-------------------------
>  1 file changed, 46 insertions(+), 46 deletions(-)
>
> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> index 52aece27b00e..af44f90379f2 100644
> --- a/mm/alloc_tag.c
> +++ b/mm/alloc_tag.c
> @@ -835,6 +835,52 @@ static int vm_module_tags_populate(void)
>         return 0;
>  }
>
> +static void release_module_tags(struct module *mod, bool used)
> +{
> +       MA_STATE(mas, &mod_area_mt, module_tags.size, module_tags.size);
> +       struct alloc_tag *start_tag;
> +       struct alloc_tag *end_tag;
> +       struct module *val;
> +
> +       mas_lock(&mas);
> +       mas_for_each_rev(&mas, val, 0)
> +               if (val == mod)
> +                       break;
> +
> +       if (!val) /* module not found */
> +               goto out;
> +
> +       if (!used)
> +               goto release_area;
> +
> +       start_tag = (struct alloc_tag *)(module_tags.start_addr + mas.index);
> +       end_tag = (struct alloc_tag *)(module_tags.start_addr + mas.last);
> +       if (!clean_unused_counters(start_tag, end_tag)) {
> +               struct alloc_tag *tag;
> +
> +               for (tag = start_tag; tag <= end_tag; tag++) {
> +                       struct alloc_tag_counters counter;
> +
> +                       if (!tag->counters)
> +                               continue;
> +
> +                       counter = alloc_tag_read(tag);
> +                       pr_info("%s:%u module %s func:%s has %llu allocated at module unload\n",
> +                               tag->ct.filename, tag->ct.lineno, tag->ct.modname,
> +                               tag->ct.function, counter.bytes);
> +               }
> +       } else {
> +               used = false;
> +       }
> +release_area:
> +       mas_store(&mas, used ? &unloaded_mod : NULL);
> +       val = mas_prev_range(&mas, 0);
> +       if (val == &prepend_mod)
> +               mas_store(&mas, NULL);
> +out:
> +       mas_unlock(&mas);
> +}
> +
>  static void *reserve_module_tags(struct module *mod, unsigned long size,
>                                  unsigned int prepend, unsigned long align)
>  {
> @@ -922,52 +968,6 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
>         return (struct alloc_tag *)(module_tags.start_addr + offset);
>  }
>
> -static void release_module_tags(struct module *mod, bool used)
> -{
> -       MA_STATE(mas, &mod_area_mt, module_tags.size, module_tags.size);
> -       struct alloc_tag *start_tag;
> -       struct alloc_tag *end_tag;
> -       struct module *val;
> -
> -       mas_lock(&mas);
> -       mas_for_each_rev(&mas, val, 0)
> -               if (val == mod)
> -                       break;
> -
> -       if (!val) /* module not found */
> -               goto out;
> -
> -       if (!used)
> -               goto release_area;
> -
> -       start_tag = (struct alloc_tag *)(module_tags.start_addr + mas.index);
> -       end_tag = (struct alloc_tag *)(module_tags.start_addr + mas.last);
> -       if (!clean_unused_counters(start_tag, end_tag)) {
> -               struct alloc_tag *tag;
> -
> -               for (tag = start_tag; tag <= end_tag; tag++) {
> -                       struct alloc_tag_counters counter;
> -
> -                       if (!tag->counters)
> -                               continue;
> -
> -                       counter = alloc_tag_read(tag);
> -                       pr_info("%s:%u module %s func:%s has %llu allocated at module unload\n",
> -                               tag->ct.filename, tag->ct.lineno, tag->ct.modname,
> -                               tag->ct.function, counter.bytes);
> -               }
> -       } else {
> -               used = false;
> -       }
> -release_area:
> -       mas_store(&mas, used ? &unloaded_mod : NULL);
> -       val = mas_prev_range(&mas, 0);
> -       if (val == &prepend_mod)
> -               mas_store(&mas, NULL);
> -out:
> -       mas_unlock(&mas);
> -}
> -
>  static int load_module(struct module *mod, struct codetag *start, struct codetag *stop)
>  {
>         /* Allocate module alloc_tag percpu counters */
> --
> 2.25.1
>

Re: [PATCH v5 2/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[Suren Baghdasaryan]

On Tue, Aug 11, 2026 at 10:41 PM Hao Ge <hao.ge@linux.dev> wrote:
>
> In reserve_module_tags(), the tag overflow check is gated on
> mem_alloc_profiling_enabled():
>
>     if (mem_alloc_profiling_enabled() && !tags_addressable())
>
> If profiling is toggled off at runtime and a module is loaded whose
> tags exceed the compressed-mode limit, shutdown_mem_profiling() is
> skipped. vm_module_tags_populate() still maps memory for the tags and
> the module loads successfully, but the total tag count now exceeds what
> NR_UNUSED_PAGEFLAG_BITS can address.
>
> Once profiling is re-enabled, ref_to_idx() computes each tag's index
> as its position in the alloc_tag array. update_page_tag_ref() masks
> it to alloc_tag_ref_mask before storing in page->flags. Indices
> beyond the mask are truncated and idx_to_ref() resolves them to wrong
> tags.
>
> This silently corrupts /proc/allocinfo: allocated pages get attributed
> to the wrong call sites, so the statistics it reports are wrong.
>
> mem_alloc_profiling_enabled() and mem_profiling_compressed are
> independent. Once compressed mode is established at boot, it stays
> active regardless of runtime toggles of mem_profiling.
>
> Remove the mem_alloc_profiling_enabled() guard. On overflow, shut down
> profiling, release the reservation, and return -EAGAIN so that
> layout_and_allocate() retries with profiling disabled: codetag sections
> are then placed as regular module data and the module loads without
> profiling rather than being rejected entirely.
>
> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> Cc: stable@vger.kernel.org
> Suggested-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Hao Ge <hao.ge@linux.dev>

I'm looking at your fixes for pre-existing issues reported by Sashiko
but this patch looks correct to me.

Acked-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  kernel/module/main.c | 25 +++++++++++++++++++++++--
>  mm/alloc_tag.c       |  8 +++++---
>  2 files changed, 28 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index 46dd8d25a605..ed26f167be84 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -2944,6 +2944,7 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
>  {
>         struct module *mod;
>         int err;
> +       unsigned long frob_size[MOD_MEM_NUM_TYPES];
>
>         /* Allow arches to frob section contents and sizes.  */
>         err = module_frob_arch_sections(info->hdr, info->sechdrs,
> @@ -2966,18 +2967,38 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
>          */
>         module_mark_ro_after_init(info->hdr, info->sechdrs, info->secstrings);
>
> +       /*
> +        * Save the sizes reserved by module_frob_arch_sections() so they can
> +        * be restored if we retry below.
> +        */
> +       for_each_mod_mem_type(type)
> +               frob_size[type] = info->mod->mem[type].size;
> +
>         /*
>          * Determine total sizes, and put offsets in sh_entsize.  For now
>          * this is done generically; there doesn't appear to be any
>          * special cases for the architectures.
>          */
> +retry:
>         layout_sections(info->mod, info);
>         layout_symtab(info->mod, info);
>
>         /* Allocate and move to the final place */
>         err = move_module(info->mod, info);
> -       if (err)
> -               return ERR_PTR(err);
> +       if (err) {
> +               if (err != -EAGAIN)
> +                       return ERR_PTR(err);
> +               /*
> +                * -EAGAIN means profiling was disabled but the module
> +                * can still load without it. Reset state and retry.
> +                */
> +               rewrite_section_headers(info, flags);
> +               for_each_mod_mem_type(type)
> +                       info->mod->mem[type].size = frob_size[type];
> +               info->sechdrs[info->index.sym].sh_flags &= ~(unsigned long)SHF_ALLOC;
> +               info->sechdrs[info->index.str].sh_flags &= ~(unsigned long)SHF_ALLOC;
> +               goto retry;
> +       }
>
>         /* Module has been copied to its final place now: return it. */
>         mod = (void *)info->sechdrs[info->index.mod].sh_addr;
> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> index af44f90379f2..0a7b657fe2de 100644
> --- a/mm/alloc_tag.c
> +++ b/mm/alloc_tag.c
> @@ -950,10 +950,12 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
>                 int grow_res;
>
>                 module_tags.size = offset + size;
> -               if (mem_alloc_profiling_enabled() && !tags_addressable()) {
> +               if (!tags_addressable()) {
>                         shutdown_mem_profiling(true);
> -                       pr_warn("With module %s there are too many tags to fit in %d page flag bits. Memory allocation profiling is disabled!\n",
> -                               mod->name, NR_UNUSED_PAGEFLAG_BITS);
> +                       pr_warn_once("With module %s there are too many tags to fit in %d page flag bits. Memory allocation profiling is disabled!\n",
> +                                    mod->name, NR_UNUSED_PAGEFLAG_BITS);
> +                       release_module_tags(mod, false);
> +                       return ERR_PTR(-EAGAIN);
>                 }
>
>                 grow_res = vm_module_tags_populate();
> --
> 2.25.1
>

Re: [PATCH v5 2/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[Petr Pavlu]

On 8/12/26 7:41 AM, Hao Ge wrote:
> In reserve_module_tags(), the tag overflow check is gated on
> mem_alloc_profiling_enabled():
> 
>     if (mem_alloc_profiling_enabled() && !tags_addressable())
> 
> If profiling is toggled off at runtime and a module is loaded whose
> tags exceed the compressed-mode limit, shutdown_mem_profiling() is
> skipped. vm_module_tags_populate() still maps memory for the tags and
> the module loads successfully, but the total tag count now exceeds what
> NR_UNUSED_PAGEFLAG_BITS can address.
> 
> Once profiling is re-enabled, ref_to_idx() computes each tag's index
> as its position in the alloc_tag array. update_page_tag_ref() masks
> it to alloc_tag_ref_mask before storing in page->flags. Indices
> beyond the mask are truncated and idx_to_ref() resolves them to wrong
> tags.
> 
> This silently corrupts /proc/allocinfo: allocated pages get attributed
> to the wrong call sites, so the statistics it reports are wrong.
> 
> mem_alloc_profiling_enabled() and mem_profiling_compressed are
> independent. Once compressed mode is established at boot, it stays
> active regardless of runtime toggles of mem_profiling.
> 
> Remove the mem_alloc_profiling_enabled() guard. On overflow, shut down
> profiling, release the reservation, and return -EAGAIN so that
> layout_and_allocate() retries with profiling disabled: codetag sections
> are then placed as regular module data and the module loads without
> profiling rather than being rejected entirely.

When the described overflow occurs, why should codetag sections be
placed as regular module data? Will the codetag support use them in any
way, or do they simply waste space? Is the issue that alloc_hooks()
creates relocations pointing into .codetag.alloc_tags?

> 
> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> Cc: stable@vger.kernel.org
> Suggested-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Hao Ge <hao.ge@linux.dev>
> ---
>  kernel/module/main.c | 25 +++++++++++++++++++++++--
>  mm/alloc_tag.c       |  8 +++++---
>  2 files changed, 28 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index 46dd8d25a605..ed26f167be84 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -2944,6 +2944,7 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
>  {
>  	struct module *mod;
>  	int err;
> +	unsigned long frob_size[MOD_MEM_NUM_TYPES];

frob_size is used to store values of module_memory::size, which has type
`unsigned int`. The types should match.

>  
>  	/* Allow arches to frob section contents and sizes.  */
>  	err = module_frob_arch_sections(info->hdr, info->sechdrs,
> @@ -2966,18 +2967,38 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
>  	 */
>  	module_mark_ro_after_init(info->hdr, info->sechdrs, info->secstrings);
>  
> +	/*
> +	 * Save the sizes reserved by module_frob_arch_sections() so they can
> +	 * be restored if we retry below.
> +	 */
> +	for_each_mod_mem_type(type)
> +		frob_size[type] = info->mod->mem[type].size;
> +
>  	/*
>  	 * Determine total sizes, and put offsets in sh_entsize.  For now
>  	 * this is done generically; there doesn't appear to be any
>  	 * special cases for the architectures.
>  	 */
> +retry:
>  	layout_sections(info->mod, info);
>  	layout_symtab(info->mod, info);
>  
>  	/* Allocate and move to the final place */
>  	err = move_module(info->mod, info);
> -	if (err)
> -		return ERR_PTR(err);
> +	if (err) {
> +		if (err != -EAGAIN)
> +			return ERR_PTR(err);

The move_module() logic is non-trivial. -EAGAIN could be returned by
other code, now or in the future.

> +		/*
> +		 * -EAGAIN means profiling was disabled but the module
> +		 * can still load without it. Reset state and retry.
> +		 */
> +		rewrite_section_headers(info, flags);
> +		for_each_mod_mem_type(type)
> +			info->mod->mem[type].size = frob_size[type];
> +		info->sechdrs[info->index.sym].sh_flags &= ~(unsigned long)SHF_ALLOC;
> +		info->sechdrs[info->index.str].sh_flags &= ~(unsigned long)SHF_ALLOC;

Why is it necessary to reset SHF_ALLOC for .symtab and .strtab here?

> +		goto retry;
> +	}
>  
>  	/* Module has been copied to its final place now: return it. */
>  	mod = (void *)info->sechdrs[info->index.mod].sh_addr;

I'm not sure this is the best approach. It's complex logic for what
appears to be an edge case related to a debugging facility. It will have
the usual problem of error paths not getting enough testing and breaking
subtly over time.

An alternative could be to reset SHF_ALLOC on the codetag section to
remove it from further processing and have relocations that point to
this section resolve to something else. It seems that alloc_hooks_tag()
could tolerate this, since it only needs to reference the associated
alloc_tag when mem_alloc_profiling_enabled() is true and that gets
disabled by reserve_module_tags() on the overflow.

It is also not an ideal approach, but I feel it could be less intrusive
to the module loader. I can put together a prototype if needed.

-- 
Thanks,
Petr