Re: [RFC PATCH] mm/damon/ops-common: call folio_test_lru() after folio_get()

[SeongJae Park <sj@kernel.org>]

On Sat, 23 May 2026 19:57:06 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Speculative folio flag access occurs before pointer re-validation, triggering VM_BUG_ON_PGFLAGS() on tail pages.
> --
> 
> commit 119755fb3390d2b7d6c7f2278054cc95e95531c6
> Author: SeongJae Park <sj@kernel.org>
> 
> mm/damon/ops-common: call folio_test_lru() after folio_get()
> 
> This patch attempts to fix a potential VM_BUG_ON_PGFLAGS crash by removing
> the speculative LRU flag check before trying to get a reference in
> damon_get_folio(). It moves the LRU check after a successful try_get.
> 
> Link: https://lore.kernel.org/20260517234112.89245-1-sj@kernel.org
> 
> > diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
> > index 3a0ddc3ac7196..d3404615f9b75 100644
> > --- a/mm/damon/ops-common.c
> > +++ b/mm/damon/ops-common.c
> > @@ -32,9 +32,9 @@ struct folio *damon_get_folio(unsigned long pfn)
> >  		return NULL;
> >  
> >  	folio = page_folio(page);
> > -	if (!folio_test_lru(folio) || !folio_try_get(folio))
> > +	if (!folio_try_get(folio))
> >  		return NULL;
> > -	if (unlikely(page_folio(page) != folio || !folio_test_lru(folio))) {
> > +	if (!folio_test_lru(folio) || unlikely(page_folio(page) != folio)) {
> 
> [Severity: High]
> Does this sequence reintroduce the same bug the patch intends to fix?
> 
> By placing the LRU flag check before the pointer re-validation, C
> short-circuit evaluation will evaluate the LRU flag first.
> 
> If the page was concurrently freed and reallocated as a tail page,
> folio_try_get() might still succeed.
> Evaluating the LRU flag on this
> unvalidated tail page will trigger the VM_BUG_ON_PGFLAGS() in
> const_folio_flags().
> 
> Should the pointer re-validation happen before accessing any folio flags in
> PFN scanners?

I was initially assume folio_try_get() will always return a valid folio.  But I
find no code to avoid Sashiko's theory in folio_try_get().

So, I think Sashiko is correct.  I will drop the above line change on the next
revision.


Thanks,
SJ

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