Re: [PATCH] mm/vma: predicate vma_assert_stabilised() on CONFIG_DEBUG_VM

[Pedro Falcato <pfalcato@suse.de>]

On Tue, Nov 10, 2026 at 03:10:40PM +0000, Lorenzo Stoakes (ARM) wrote:
> I introduced commit 17fd82c3abe0 ("mm/vma: add and use
> vma_assert_stabilised()") with the intent of picking up programmatic bugs
> when debugging mm code, but kept the general practice of asserting even
> when CONFIG_DEBUG_VM is not set in line with existing code.
> 
> However, it turns out doing so can cause performance regressions,
> especially when asserted in vma_has_anon_rmap() which is referenced in a
> number of hotpaths.
> 
> Since the intent was always a debug-only assertion, explicitly check for
> CONFIG_DEBUG_VM and do not assert if it isn't set.
> 
> Really all of the assert paths need to be predicated on this, but start
> with this one only, out of caution.
> 
> Additionally, since a detached VMA is trivially stabilised (nothing to
> stabilise against), check for this case at the outset, and update
> vma_has_anon_rmap() to not explicitly need to check for this.
> 
> This also, importantly, avoids reading any VMA fields if CONFIG_DEBUG_VM is
> not set.
> 
> Target the commit which introduced vma_has_anon_rmap() for the fix, as this
> is where the issue started manifesting in practice.
> 
> Fixes: 38564710f443 ("mm: implement and use vma_has_anon_rmap(), silence KCSAN")
> Reported-by: Seunghyun An <san55@wisc.edu>
> Closes: https://lore.kernel.org/linux-mm/20261110000002.1834454-1-san55@wisc.edu/
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Pedro Falcato <pfalcato@suse.de>

-- 
Pedro