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    SubjectRe: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT
    On Wed 13-06-18 08:32:19, Vlastimil Babka wrote:
    > On 06/12/2018 04:11 PM, Jason Baron wrote:
    > >
    > >
    > > On 06/12/2018 03:46 AM, Michal Hocko wrote:
    > >> On Mon 11-06-18 12:23:58, Jason Baron wrote:
    > >>> On 06/11/2018 11:03 AM, Michal Hocko wrote:
    > >>>> So can we start discussing whether we want to allow MADV_DONTNEED on
    > >>>> mlocked areas and what downsides it might have? Sure it would turn the
    > >>>> strong mlock guarantee to have the whole vma resident but is this
    > >>>> acceptable for something that is an explicit request from the owner of
    > >>>> the memory?
    > >>>>
    > >>>
    > >>> If its being explicity requested by the owner it makes sense to me. I
    > >>> guess there could be a concern about this breaking some userspace that
    > >>> relied on MADV_DONTNEED not freeing locked memory?
    > >>
    > >> Yes, this is always the fear when changing user visible behavior. I can
    > >> imagine that a userspace allocator calling MADV_DONTNEED on free could
    > >> break. The same would apply to MLOCK_ONFAULT/MCL_ONFAULT though. We
    > >> have the new flag much shorter so the probability is smaller but the
    > >> problem is very same. So I _think_ we should treat both the same because
    > >> semantically they are indistinguishable from the MADV_DONTNEED POV. Both
    > >> remove faulted and mlocked pages. Mlock, once applied, should guarantee
    > >> no later major fault and MADV_DONTNEED breaks that obviously.
    >
    > I think more concerning than guaranteeing no later major fault is
    > possible data loss, e.g. replacing data with zero-filled pages.

    But MADV_DONTNEED is an explicit call for data loss. Or do I miss your
    point?

    > The madvise manpage is also quite specific about not allowing
    > MADV_DONTNEED and MADV_FREE for locked pages.

    Yeah, but that seems to describe the state of the art rather than
    explain why.

    > So I don't think we should risk changing that for all mlocked pages.
    > Maybe we can risk MCL_ONFAULT, since it's relatively new and has few users?

    That is what Jason wanted but I argued that the two are the same from
    MADV_DONTNEED point of view. I do not see how treating them differently
    would be less confusing or error prone. It's new so we can make it
    behave differently is certainly not an argument.

    > >> So the more I think about it the more I am worried about this but I am
    > >> more and more convinced that making ONFAULT special is just a wrong way
    > >> around this.
    > >>
    > >
    > > Ok, I share the concern that there is a chance that userspace is relying
    > > on MADV_DONTNEED not free'ing locked memory. In that case, what if we
    > > introduce a MADV_DONTNEED_FORCE, which does everything that
    > > MADV_DONTNEED currently does but in addition will also free mlock areas.
    > > That way there is no concern about breaking something.
    >
    > A new niche case flag? Sad :(
    >
    > BTW I didn't get why we should allow this for MADV_DONTNEED but not
    > MADV_FREE. Can you expand on that?

    Well, I wanted to bring this up as well. I guess this would require some
    more hacks to handle the reclaim path correctly because we do rely on
    VM_LOCK at many places for the lazy mlock pages culling.

    --
    Michal Hocko
    SUSE Labs

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