Re: [PATCH v2 2/2] mm/madvise: allow guard page install/remove under VMA lock

[David Hildenbrand (Red Hat)]

On 10.11.25 18:22, Lorenzo Stoakes wrote:
> We only need to keep the page table stable so we can perform this operation
> under the VMA lock. PTE installation is stabilised via the PTE lock.
>
> One caveat is that, if we prepare vma->anon_vma we must hold the mmap read
> lock. We can account for this by adapting the VMA locking logic to
> explicitly check for this case and prevent a VMA lock from being acquired
> should it be the case.
>
> This check is safe, as while we might be raced on anon_vma installation,
> this would simply make the check conservative, there's no way for us to see
> an anon_vma and then for it to be cleared, as doing so requires the
> mmap/VMA write lock.
>
> We abstract the VMA lock validity logic to is_vma_lock_sufficient() for
> this purpose, and add prepares_anon_vma() to abstract the anon_vma logic.
>
> In order to do this we need to have a way of installing page tables
> explicitly for an identified VMA, so we export walk_page_range_vma() in an
> unsafe variant - walk_page_range_vma_unsafe() and use this should the VMA
> read lock be taken.
>
> We additionally update the comments in madvise_guard_install() to more
> accurately reflect the cases in which the logic may be reattempted,
> specifically THP huge pages being present.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---

Looks good to me

Acked-by: David Hildenbrand (Red Hat) <david@xxxxxxxxxx>

--
Cheers

David