Re: [RFC 1/1] mm/madvise: fail MADV_PAGEOUT on VM_DROPPABLE VMA

[Lance Yang]

Hi Barry, David and Lorenzo,

Thanks a lot for taking time to review!

On Mon, Jan 20, 2025 at 6:45 PM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> Sorry but NACK again on this :(
>
> On Mon, Jan 20, 2025 at 09:30:38AM +0800, Lance Yang wrote:
> > MADV_PAGEOUT should fail on VMAs with the VM_DROPPABLE flag. While
> > MADV_PAGEOUT is intended to move anonymous pages to swap, VM_DROPPABLE
> > should not be swapped out.
> >
> > There is an issue where using MADV_PAGEOUT on a VMA with the VM_DROPPABLE
> > flag behaves like MADV_DONTNEED, causing the pages to be dropped. This
> > could break the semantics of MADV_PAGEOUT, IMO.
> >
> > So, let's add a check to detect the VM_DROPPABLE flag before doing
> > MADV_PAGEOUT and returns -EINVAL.
>
> No, let's not.

I think I completely got it wrong. Learning so much from your patient responses!

>
> Firstly this behaviour, whether you feel it is right or not, is now
> _established userland behaviour_. A pedantic interpretation of how
> MADV_PAGEOUT ought to interact with MAP_DROPPABLE doesn't trump what is
> already in released kernel versions, unless it is wrong in a -broken-
> fashion.

Yeah, this patch would break the established userland behaviour as well.

>
> Also I'd say you'd 100% expect an anon page to be dropped in the case of
> MAP_DROPPABLE, the semantics of which are 'if there is a request to drop
> this, drop it'. Here, the user is saying 'hey drop this'. So we drop it :)

Yep, you're right.

>
> I think the more correct patch would be to extend the man page to
> explicitly mention MAP_DROPPABLE.

Thanks again for your time!
Lance

>
> >
> > Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
> > Signed-off-by: Mingzhe Yang <mingzhe.yang@xxxxxx>
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > ---
> >  mm/madvise.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 49f3a75046f6..29d0234da8a1 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1263,6 +1263,8 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
> >       case MADV_COLD:
> >               return madvise_cold(vma, prev, start, end);
> >       case MADV_PAGEOUT:
> > +             if (vma->vm_flags & VM_DROPPABLE)
> > +                     return -EINVAL;
> >               return madvise_pageout(vma, prev, start, end);
> >       case MADV_FREE:
> >       case MADV_DONTNEED:
> > --
> > 2.45.2
> >