Re: [PATCH 1/3] mm: introduce VM_MAYBE_GUARD and make visible for guard regions

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Oct 30, 2025 at 2:48 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 10/30/25 20:47, Lorenzo Stoakes wrote:
> > On Thu, Oct 30, 2025 at 07:47:34PM +0100, Vlastimil Babka wrote:
> >> >
> >> > Could we use MADVISE_VMA_READ_LOCK mode (would be actually an improvement
> >> > over the current MADVISE_MMAP_READ_LOCK), together with the atomic flag
> >> > setting? I think the places that could race with us to cause RMW use vma
> >> > write lock so that would be excluded. Fork AFAICS unfortunately doesn't (for
> >> > the oldmm) and it probably would't make sense to start doing it. Maybe we
> >> > could think of something to deal with this special case...
> >>
> >> During discussion with Pedro off-list I realized fork takes mmap lock for
> >> write on the old mm, so if we kept taking mmap sem for read, then vma lock
> >> for read in addition (which should be cheap enough, also we'd only need it
> >> in case VM_MAYBE_GUARD is not yet set), and set the flag atomicaly, perhaps
> >> that would cover all non-bening races?
> >>
> >>
> >
> > We take VMA write lock in dup_mmap() on each mpnt (old VMA).
>
> Ah yes I thought it was the new one.
>
> > We take the VMA write lock (vma_start_write()) for each mpnt.
> >
> > We then vm_area_dup() the mpnt to the new VMA before calling:
> >
> > copy_page_range()
> > -> vma_needs_copy()
> >
> > Which is where the check is done.
> >
> > So we are holding the VMA write lock, so a VMA read lock should suffice no?
>
> Yeah, even better!
>
> > For belts + braces we could atomically read the flag in vma_needs_copy(),
> > though note it's intended VM_COPY_ON_FORK could have more than one flag.
> >
> > We could drop that for now and be explicit.
>
> Great!

Overall, I think it should be possible to set this flag atomically
under VMA read-lock. However, if you introduce new vm_flags
manipulation functions, please make sure they can't be used for other
vm_flags. In Android I've seen several "interesting" attempts to
update vm_flags under a read-lock (specifically in the page-fault
path) and had to explain why that's a bad idea.