Re: [PATCH 1/2] powerpc/64s/radix: don't dedup vmemmap covered by an existing PMD leaf

[Muchun Song <muchun.song@linux.dev>]

Hi Ritesh,

> On Oct 27, 2026, at 11:22, Ritesh Harjani (IBM) <ritesh.list@gmail.com> wrote:
> 
> radix__vmemmap_populate() is asked for one 16MB section, which is 16KB of
> struct pages. With no altmap it rounds the request down to a PMD leaf.
> That leaf describes 2GB of PFNs with 64KB pages (128MB with 4KB pages),
> so it also covers sections that are not present yet. It cannot be freed
> while any of those sections are still in use.
> 
> A devdax namespace that lands in the same window is marked for vmemmap dedup.
> vmemmap_populate_compound_pages() treated the leaf as an existing huge mapping
> and skipped it. The core then initializes only the head vmemmap page and assumes
> the rest is the shared tail. Those remaining tail slots are real struct pages
> still holding the page_init_poison() pattern and are not the shared tail pages.

Right.

> __init_single_page() warns on the zone mismatch and the following pageblock init
> faults in __set_pfnblock_flags_mask().

Yeah, the new adding warning is useful.

> 
> If the section's vmemmap is already a PMD leaf, clear its compound order

I think it's enough to fix this issue.

> and reuse the leaf, so every struct page in the section is initialized.
> The shared-tail layout needs base-page PTEs, so it cannot be built on top of
> this leaf and dropping the leaf is not an option while the neighbouring range
> is still using it. So, fail the compound populate if a leaf is still seen there.
> 
> <Call Trace>
> ============
> [    3.467060][    T1] WARNING: mm/mm_init.c:600 at __init_single_page+0xd0/0xf4, CPU#0: swapper/0/1
> <...>
> [    3.477419][    T1] NIP [c000000000576b30] __init_single_page+0xd0/0xf4
> [    3.477827][    T1] LR [c0000000005778b0] __init_zone_device_page+0x30/0xb8
> [    3.478444][    T1] Call Trace:
> [    3.478764][    T1]  0x10000 (unreliable)
> [    3.479874][    T1]  memmap_init_zone_device+0x164/0x430
> [    3.480392][    T1]  memremap_pages+0x43c/0x93c
> [    3.480836][    T1]  devm_memremap_pages+0x3c/0xa4
> [    3.481328][    T1]  dev_dax_probe+0x264/0x3d0
> [    3.481817][    T1]  dax_bus_probe+0x140/0x1c8
> [    3.482237][    T1]  really_probe+0x108/0x580
> [    3.482681][    T1]  __driver_probe_device+0xcc/0x284
> [    3.483186][    T1]  driver_probe_device+0x5c/0x108
> [    3.483656][    T1]  __device_attach_driver+0x10c/0x1e8
> [    3.484134][    T1]  bus_for_each_drv+0xc0/0x13c
> <...>
> [    3.499972][    T1] BUG: Kernel NULL pointer dereference on read at 0x00001400
> [    3.500417][    T1] Faulting instruction address: 0xc0000000005f4ecc
> [    3.500417][    T1] Faulting instruction address: 0xc0000000005f4ecc
> cpu 0x0: Vector: 300 (Data Access) at [c000000006756b50]
>    pc: c0000000005f4ecc: __set_pfnblock_flags_mask.constprop.0+0x34/0x160
>    lr: c000000000577f38: memmap_init_zone_device+0x358/0x430
>    pid   = 1, comm = swapper/0
> enter ? for help
>  memmap_init_zone_device+0x358/0x430
>  memmap_init_zone_device+0x220/0x430 (unreliable)
>  memremap_pages+0x43c/0x93c
>  devm_memremap_pages+0x3c/0xa4
>  dev_dax_probe+0x264/0x3d0
>  dax_bus_probe+0x140/0x1c8
>  really_probe+0x108/0x580
> <...>
> 
> Fixes: bd0862382adb ("mm/sparse-vmemmap: drop Device DAX-specific population path")

Actually, the right Fixes is:

Fixes: f2b79c0d7968 ("powerpc/book3s64/radix: add support for vmemmap optimization for radix")

On older kernels, these errors are silent. They trigger a kernel panic when the user
actually uses Device DAX, rather than during the initialization phase. 

So, for older kernels, we should guide them to backport the patch.

> Reported-by: Pavithra <pavrampu@linux.ibm.com>
> Assisted-by: LLM
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
> Fixes tag here is added since the fix is dependent upon the change mentioned in
> "Fixes" tag. The Fixes commit-id is from mm-new branch. Although this could be
> an older pre-existing issue, but got exposed after this patch series.

I knew this issue, so I have a local commit to fix it as well. I'd like to share
it with you, because it looks a little simple, does it work for you?

powerpc/mm: fix uninitialized ZONE_DEVICE struct pages with PMDs

PowerPC radix vmemmap population can reuse an existing PMD mapping while
populating a device DAX range whose section is marked as HVO-enabled. The
reused mapping is an ordinary vmemmap mapping: the redundant tail struct
pages are not remapped to the shared tail page.

Leaving the section marked as HVO-enabled makes later ZONE_DEVICE
initialization treat the range as optimized and initialize only the struct
pages that would be backed by a real HVO mapping. The remaining tail struct
pages can then be left uninitialized even though they are still used as
metadata for the device DAX compound page.

The Fixes target predates the section-based vmemmap optimization
infrastructure, so older kernels do not have section state that can be
marked non-HVO. Backports to such kernels need to handle the PMD-reuse
case in powerpc radix code directly, for example by initializing the tail
struct pages that core ZONE_DEVICE initialization would otherwise skip.

Mark the section as non-HVO when reusing an existing PMD mapping, so the
section's HVO state matches the actual vmemmap layout and all required
struct pages are initialized.

Fixes: f2b79c0d7968 ("powerpc/book3s64/radix: add support for vmemmap optimization for radix")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 arch/powerpc/mm/book3s64/radix_pgtable.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
index 72793ace1fb6..fc3a2d4b422d 100644
--- a/arch/powerpc/mm/book3s64/radix_pgtable.c
+++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -1200,7 +1200,16 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
 			return -ENOMEM;

 		if (pmd_leaf(READ_ONCE(*pmd))) {
-			/* existing huge mapping. Skip the range */
+			unsigned long addr_pfn = page_to_pfn((struct page *)addr);
+			struct mem_section *ms = __pfn_to_section(addr_pfn);
+
+			/*
+			 * Existing PMD mappings can cover the edges of a valid
+			 * HVO populate range.  They are ordinary vmemmap mappings,
+			 * so the current section must not be treated as HVO-backed
+			 * when its struct pages are initialized later.
+			 */
+			section_set_order(ms, 0);
 			next = pmd_addr_end(addr, end);
 			continue;
 		}