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

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 10 Nov 2025, 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.

Makes sense.

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>