linux-kernel - Re: [PATCH v3 5/8] mm: make folio_lock_fault indicate the state of mmap

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Date:   Tue, 27 Jun 2023 09:01:06 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, hannes@...xchg.org,
        mhocko@...e.com, josef@...icpanda.com, jack@...e.cz,
        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        michel@...pinasse.org, liam.howlett@...cle.com, jglisse@...gle.com,
        vbabka@...e.cz, minchan@...gle.com, dave@...olabs.net,
        punit.agrawal@...edance.com, lstoakes@...il.com, hdanton@...a.com,
        peterx@...hat.com, ying.huang@...el.com, david@...hat.com,
        yuzhao@...gle.com, dhowells@...hat.com, hughd@...gle.com,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        pasha.tatashin@...een.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 5/8] mm: make folio_lock_fault indicate the state of
 mmap_lock upon return

On Tue, Jun 27, 2023 at 1:09 AM Alistair Popple <apopple@...dia.com> wrote:
>
>
> Suren Baghdasaryan <surenb@...gle.com> writes:
>
> > folio_lock_fault might drop mmap_lock before returning and to extend it
> > to work with per-VMA locks, the callers will need to know whether the
> > lock was dropped or is still held. Introduce new fault_flag to indicate
> > whether the lock got dropped and store it inside vm_fault flags.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > ---
> >  include/linux/mm_types.h | 1 +
> >  mm/filemap.c             | 2 ++
> >  2 files changed, 3 insertions(+)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 79765e3dd8f3..6f0dbef7aa1f 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -1169,6 +1169,7 @@ enum fault_flag {
> >       FAULT_FLAG_UNSHARE =            1 << 10,
> >       FAULT_FLAG_ORIG_PTE_VALID =     1 << 11,
> >       FAULT_FLAG_VMA_LOCK =           1 << 12,
> > +     FAULT_FLAG_LOCK_DROPPED =       1 << 13,
>
> Minor nit but this should also be added to the enum documentation
> comment above this.

Thanks! Sounds like we will be dripping the new flag, so hopefully I
won't need to document it :)

>
> >  };
> >
> >  typedef unsigned int __bitwise zap_flags_t;
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 87b335a93530..8ad06d69895b 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -1723,6 +1723,7 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
> >                       return VM_FAULT_RETRY;
> >
> >               mmap_read_unlock(mm);
> > +             vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
> >               if (vmf->flags & FAULT_FLAG_KILLABLE)
> >                       folio_wait_locked_killable(folio);
> >               else
> > @@ -1735,6 +1736,7 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
> >               ret = __folio_lock_killable(folio);
> >               if (ret) {
> >                       mmap_read_unlock(mm);
> > +                     vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
> >                       return VM_FAULT_RETRY;
> >               }
> >       } else {
>