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

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

[Lance Yang]

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.

Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
---
 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

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

[Barry Song]

On Mon, Jan 20, 2025 at 2:31 PM Lance Yang <ioworker0@gmail.com> 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.
>
> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
> Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
> Signed-off-by: Lance Yang <ioworker0@gmail.com>

I am not convinced this is a correct patch. try_to_unmap_one() will
drop the folio.
madvise_pageout(vma, prev, start, end) will still go to shrink_folio_list() and
try_to_unmap_one().

> ---
>  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
>

Thanks
Barry

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

[David Hildenbrand]

On 20.01.25 02:30, 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.

It behaves exactly correct I think. They can be dropped any time, which 
includes on misplaced MADV_PAGEOUT.

-- 
Cheers,

David / dhildenb

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

[Lorenzo Stoakes]

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.

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.

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 :)

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

>
> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
> Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
> Signed-off-by: Lance Yang <ioworker0@gmail.com>
> ---
>  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
>

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@oracle.com> 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@ly.com>
> > Signed-off-by: Lance Yang <ioworker0@gmail.com>
> > ---
> >  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
> >

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

[David Hildenbrand]

On 20.01.25 14:47, Lance Yang wrote:
> 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@oracle.com> 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!

Maybe you can make the documentation (man pages?) clearer in that 
regard? Thanks!

-- 
Cheers,

David / dhildenb

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

[Lance Yang]

On Mon, Jan 20, 2025 at 9:57 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 20.01.25 14:47, Lance Yang wrote:
> > 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@oracle.com> 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!
>
> Maybe you can make the documentation (man pages?) clearer in that
> regard? Thanks!

Of course, I will give it a try ASAP ;)

Thanks for your suggestion!
Lance

>
> --
> Cheers,
>
> David / dhildenb
>