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

[Lorenzo Stoakes]

On Mon, Nov 10, 2025 at 04:44:45PM +0100, David Hildenbrand (Red Hat) wrote:
> On 09.11.25 12:16, 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_valid() 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.
> >
> > Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > ---
>
> [...]
>
> >
> > +/* Does this operation invoke anon_vma_prepare()? */
> > +static bool prepares_anon_vma(int behavior)
> > +{
> > +	switch (behavior) {
> > +	case MADV_GUARD_INSTALL:
> > +		return true;
> > +	default:
> > +		return false;
> > +	}
> > +}
> > +
> > +/*
> > + * We have acquired a VMA read lock, is the VMA valid to be madvise'd under VMA
> > + * read lock only now we have a VMA to examine?
> > + */
> > +static bool is_vma_lock_valid(struct vm_area_struct *vma,
> > +		struct madvise_behavior *madv_behavior)
>
>
> Not sure about the "valid" terminology here.
>
> Would "is_vma_lock_sufficient" be a better name, that would imply when
> "false" that another lock is required, because the VMA lock is insufficient?

OK makes sense, will change

>
>
> --
> Cheers
>
> David

Cheers, Lorenzo