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

[SeongJae Park <sj@kernel.org>]

On Sat, 23 May 2026 13:26:07 -0700 SeongJae Park <sj@kernel.org> wrote:

> 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.

But, I may still mark the folio_test_lru() as no more unlikely.


Thanks,
SJ

[...]