Re: [PATCH v10 2/6] alloc_tag: clean up the populate failure path

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Sep 28, 2026 at 01:09:13PM +0200, Uladzislau Rezki wrote:
> On Thu, Sep 17, 2026 at 10:32:48AM +0800, Hao Ge wrote:
> > Hi Suren、David、Lorenzo
> > 
> > On 2026/9/17 09:12, Hao Ge wrote:
> > > Hi Suren
> > > 
> > > On 2026/9/17 03:07, Suren Baghdasaryan wrote:
> > >> On Wed, Sep 16, 2026 at 9:31 AM Suren Baghdasaryan <surenb@google.com> wrote:
> > >>>
> > >>> On Tue, Sep 15, 2026 at 11:00 PM Hao Ge <hao.ge@linux.dev> wrote:
> > >>>>
> > >>>> Hi Suren
> > >>>>
> > >>>> Thanks for you review.
> > >>>>
> > >>>> On 2026/9/16 05:09, Suren Baghdasaryan wrote:
> > >>>>> On Mon, Sep 14, 2026 at 11:59 PM Hao Ge <hao.ge@linux.dev> wrote:
> > >>>>>>
> > >>>>>> The reservation is already stored in the maple tree when
> > >>>>>> vm_module_tags_populate() fails. A failed load never unloads the
> > >>>>>> module, so nothing releases the entry. Release it and roll
> > >>>>>> module_tags.size back. Without the rollback a concurrent load that
> > >>>>>> already passed needs_section_mem() can reuse the freed gap, skip
> > >>>>>> vm_module_tags_populate() and write to unmapped memory.
> > >>>>>>
> > >>>>>> vmap_pages_range() may have installed some PTEs before failing. A
> > >>>>>> retry to populate the same range would BUG on them, so undo them,
> > >>>>>> but only if vmap actually ran.
> > >>>>>>
> > >>>>>> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> > >>>>>> Fixes: 0f9b685626da ("alloc_tag: populate memory for module tags as needed")
> > >>>>>
> > >>>>> If a patch fixes two other patches, it likely needs to be split so
> > >>>>> that each part fixes one patch only. But read on please. I'm not sure
> > >>>>> if one of the fixes is correct.
> > >>>>>
> > >>>>
> > >>>> Kept them in one patch because they only make sense together.
> > >>>>
> > >>>> After a failed populate the reservation is released and the
> > >>>> size rolled back, so the next load takes the same gap and calls
> > >>>> vm_module_tags_populate() again, and that retry would trip over
> > >>>> the PTEs the failed vmap left behind.
> > >>>>
> > >>>> Without the rollback a load that already passed needs_section_mem()
> > >>>> just reuses the gap, skips populate and writes to unmapped memory.
> > >>>> So splitting them leaves each half broken.
> > >>>
> > >>> Yeah, that's why I think vmap_pages_range() cleanup should be a
> > >>> completely separate patch with vmap_pages_range() doing cleanup
> > >>> itself.
> > >>>
> > > 
> > > I think I get where you're coming from now.
> > > Thanks for taking the time to explain.
> > > 
> > >>>>
> > >>>>>> Reported-by: Sashiko <sashiko-bot@kernel.org>
> > >>>>>> Cc: stable@vger.kernel.org
> > >>>>>> Signed-off-by: Hao Ge <hao.ge@linux.dev>
> > >>>>>> ---
> > >>>>>>  mm/alloc_tag.c | 10 ++++++++++
> > >>>>>>  1 file changed, 10 insertions(+)
> > >>>>>>
> > >>>>>> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> > >>>>>> index 2070e682fe10..95ddf5b743d0 100644
> > >>>>>> --- a/mm/alloc_tag.c
> > >>>>>> +++ b/mm/alloc_tag.c
> > >>>>>> @@ -804,6 +804,13 @@ static int vm_module_tags_populate(void)
> > >>>>>>                                      next_page, PAGE_SHIFT) < 0) {
> > >>>>>>                         release_pages_arg arg = { .pages = next_page };
> > >>>>>>
> > >>>>>> +                       /*
> > >>>>>> +                        * vmap_pages_range() only runs once all pages were
> > >>>>>> +                        * allocated, and it may have installed some mappings
> > >>>>>> +                        * before failing. Undo them.
> > >>>>>
> > >>>>> If vmap_pages_range() failed to map the range, why should we need to
> > >>>>> undo it? If it indeed leaves some partial mapping when failing then I
> > >>>>> would argue that vmap_pages_range() should be undoing these partial
> > >>>>> mappings itself before returning the error.
> > >>>>>
> > >>>> Yeah it does - none of the paths under vmap_pages_range() undo themselves on error.
> > >>>
> > >>> Hmm. I wonder if this behavior is intentional or requires a fix. The
> > >>> only possible reason I can think of is performance but I can't imagine
> > >>> a failure to map is a performance-critical case.
> > >>>
> > >>> +David Hildenbrand (Red Hat), +Lorenzo Stoakes (Oracle) what do you
> > >>
> > >> Huh, my gmail thinks you are still at your previous companies :)
> > >>
> > > 
> > > I manually fixed it on the mailing list for this thread.
> > > 
> > >>> guys think? Shouldn't vmap_pages_range() undo its possible partial
> > >>> mapping when it fails?
> > >>>
> > >>>
> > >>>> pcpu_map_pages does perform cleanup on its own as well. There may be other similar sites.
> > >>>> https://elixir.bootlin.com/linux/v7.3-rc3/source/mm/percpu-vm.c#L255
> > >>>> so I did the same here.
> > >>>>
> > >>>> If we really want vmap_pages_range() to handle this cleanup internally, I believe that would
> > >>>> be a relatively large change, and we would need to consider many more cases. So I think we keep
> > >>>> the current behaviour for now.
> > >>>
> > >>> Yeah, I understand it would require a deeper cut but if that's the
> > >>> right way to handle this we might as well do that. Let's see what MM
> > >>> folks think.
> > >>>
> > > 
> > > Sounds good to me. Let's wait and hear what the MM folks say.
> > > 
> > 
> > I just looked into the call sites for __vmap_pages_range.
> > And I found there's a bug in one vmalloc caller [1].
> > When gfp_mask has __GFP_NOFAIL, the code keeps retrying
> > __vmap_pages_range. If there are leftover PTE mappings,
> > this can trigger a BUG(); [2]
> > https://elixir.bootlin.com/linux/v7.3-rc3/source/mm/vmalloc.c#L3946 [1]
> > https://elixir.bootlin.com/linux/v7.3-rc3/source/mm/vmalloc.c#L121  [2]
> > 
> Indeed the below code is incomplete:
> 
> <snip>
> 	/*
> 	 * page tables allocations ignore external gfp mask, enforce it
> 	 * by the scope API
> 	 */
> 	flags = memalloc_apply_gfp_scope(gfp_mask);
> 	do {
> 		ret = __vmap_pages_range(addr, addr + size, prot, area->pages,
> 				page_shift, nested_gfp);
> 		if (nofail && (ret < 0))
> 			schedule_timeout_uninterruptible(1);
> 	} while (nofail && (ret < 0));
> 	memalloc_restore_scope(flags);
> <snip>
> 
> and it should be fixed. As the __vmap_pages_range() can actually fail
> and leave some installed mappings, we have to perform some cleanups
> on failure or on repeat(if nofail is true).
> 
> Below code:
> 
> +			/*
> +			 * vmap_pages_range() only runs once all pages were
> +			 * allocated, and it may have installed some mappings
> +			 * before failing. Undo them.
> +			 */
> +			if (nr == more_pages)
> +				vunmap_range(phys_end, phys_end + (nr << PAGE_SHIFT));
> 
> we can use but we need to check if partially mapped space is not an
> issue for the vunmap_range(). And it looks like it is not.
> 
> <snip>
> @@ -3953,8 +3990,15 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>         do {
>                 ret = __vmap_pages_range(addr, addr + size, prot, area->pages,
>                                 page_shift, nested_gfp);
> -               if (nofail && (ret < 0))
> -                       schedule_timeout_uninterruptible(1);
> +               if (ret < 0) {
> +                       /*
> +                        * __vmap_pages_range() may have installed some mappings
> +                        * before failing. Undo them before returning or retrying.
> +                        */
> +                       vunmap_range(addr, addr + size);
> +                       if (nofail)
> +                               schedule_timeout_uninterruptible(1);
> +               }
>         } while (nofail && (ret < 0));
>         memalloc_restore_scope(flags);
> <snip>
> 
And looking further. If the __vmap_pages_range()/__vmap_pages_range_noflush()
and friends fail, a proper cleanups should be done inside, IMO.

But i am checking callers and see that some do some not:

<snip hooks.c>
	__vunmap_range_noflush(
		vmalloc_shadow(start),
		vmalloc_shadow(start + clean * PAGE_SIZE));
	__vunmap_range_noflush(
		vmalloc_origin(start),
		vmalloc_origin(start + clean * PAGE_SIZE));
<snip hooks.c>

kmsan_ioremap_page_range() it does clean ups;
kmsan_vmap_pages_range_noflush() looks like does not care in shadow.c;
__vmalloc_area_node() as noted, it is broken as there is a BUG() on next try if nofail;
pcpu_map_pages() it does cleanups.
...

and so on.

--
Uladzislau Rezki