Re: [PATCH V7 0/2] mm: shmem: support POSIX_FADV_[WILL|DONT]NEED for shmem files

[Frank van der Linden]

On Tue, Apr 18, 2023 at 9:19 PM Pavan Kondeti <quic_pkondeti@xxxxxxxxxxx> wrote:
>
> On Tue, Apr 18, 2023 at 05:29:42PM +0000, Frank van der Linden wrote:
> > Below is a quick patch to allow FADVISE_DONTNEED for shmem to reclaim
> > mapped pages too. This would fit our usecase, and matches MADV_PAGEOUT
> > more closely.
> >
> > The patch series as posted skips mapped pages even if you remove
> > the folio_mapped() check, because page_referenced() in
> > shrink_page_list() will find page tables with the page mapped,
> > and ignore_references is not set when called from reclaim_pages().
> >
> > You can make this work in a similar fashion to MADV_PAGEOUT by
> > first unmapping a page, but only if the mapping belongs to
> > the caller. You just have to change the check for "is there
> > only one mapping and am I the owner". To do that, change a few
> > lines in try_to_unmap to allow for checking the mm the mapping
> > belongs to, and pass in current->mm (NULL still unmaps all mappings).
> >
> > I lightly tested this in a somewhat older codebase, so the diff
> > below isn't fully tested. But if there are no objections to
> > this approach, we could add it on top of your patchset after
> > better testing.
> >
> > - Frank
> >
> > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > index b87d01660412..4403cc2ccc4c 100644
> > --- a/include/linux/rmap.h
> > +++ b/include/linux/rmap.h
> > @@ -368,6 +368,8 @@ int folio_referenced(struct folio *, int is_locked,
> >
> >  void try_to_migrate(struct folio *folio, enum ttu_flags flags);
> >  void try_to_unmap(struct folio *, enum ttu_flags flags);
> > +void try_to_unmap_mm(struct mm_struct *mm, struct folio *folio,
> > +                     enum ttu_flags flags);
> >
> >  int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
> >                               unsigned long end, struct page **pages,
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 8632e02661ac..4d30e8f5afe2 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1443,6 +1443,11 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
> >       munlock_vma_folio(folio, vma, compound);
> >  }
> >
> > +struct unmap_arg {
> > +     enum ttu_flags flags;
> > +     struct mm_struct *mm;
> > +};
> > +
> >  /*
> >   * @arg: enum ttu_flags will be passed to this argument
> >   */
> > @@ -1455,7 +1460,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >       struct page *subpage;
> >       bool anon_exclusive, ret = true;
> >       struct mmu_notifier_range range;
> > -     enum ttu_flags flags = (enum ttu_flags)(long)arg;
> > +     struct unmap_arg *uap = (struct unmap_arg *)arg;
> > +     enum ttu_flags flags = uap->flags;
> > +
> > +     if (uap->mm && uap->mm != mm)
> > +             return true;
> >
> >       /*
> >        * When racing against e.g. zap_pte_range() on another cpu,
> > @@ -1776,6 +1785,7 @@ static int folio_not_mapped(struct folio *folio)
> >
> >  /**
> >   * try_to_unmap - Try to remove all page table mappings to a folio.
> > + * @mm: mm to unmap from (NULL to unmap from all)
> >   * @folio: The folio to unmap.
> >   * @flags: action and flags
> >   *
> > @@ -1785,11 +1795,16 @@ static int folio_not_mapped(struct folio *folio)
> >   *
> >   * Context: Caller must hold the folio lock.
> >   */
> > -void try_to_unmap(struct folio *folio, enum ttu_flags flags)
> > +void try_to_unmap_mm(struct mm_struct *mm, struct folio *folio,
> > +             enum ttu_flags flags)
> >  {
> > +     struct unmap_arg ua = {
> > +             .flags = flags,
> > +             .mm = mm,
> > +     };
> >       struct rmap_walk_control rwc = {
> >               .rmap_one = try_to_unmap_one,
> > -             .arg = (void *)flags,
> > +             .arg = (void *)&ua,
> >               .done = folio_not_mapped,
> >               .anon_lock = folio_lock_anon_vma_read,
> >       };
> > @@ -1800,6 +1815,11 @@ void try_to_unmap(struct folio *folio, enum ttu_flags flags)
> >               rmap_walk(folio, &rwc);
> >  }
> >
> > +void try_to_unmap(struct folio *folio, enum ttu_flags flags)
> > +{
> > +     try_to_unmap_mm(NULL, folio, flags);
> > +}
> > +
> >  /*
> >   * @arg: enum ttu_flags will be passed to this argument.
> >   *
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 1af85259b6fc..b24af2fb3378 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -2362,8 +2362,24 @@ static void shmem_isolate_pages_range(struct address_space *mapping, loff_t star
> >
> >               if (!folio_try_get(folio))
> >                       continue;
> > -             if (folio_test_unevictable(folio) || folio_mapped(folio) ||
> > -                             folio_isolate_lru(folio)) {
> > +
> > +             if (folio_test_unevictable(folio)) {
> > +                     folio_put(folio);
> > +                     continue;
> > +             }
> > +
> > +             /*
> > +              * If the folio is mapped once, try to unmap it from the
> > +              * caller's page table. If it's still mapped afterwards,
> > +              * it belongs to someone else, and we're not going to
> > +              * change someone else's mapping.
> > +              */
> > +             if (folio_mapcount(folio) == 1 && folio_trylock(folio)) {
> > +                     try_to_unmap_mm(current->mm, folio, TTU_BATCH_FLUSH);
> > +                     folio_unlock(folio);
> > +             }
>
> Is rmap walk can be done from a RCU read critical section which does not
> allow explicit blocking?
>
> Thanks,
> Pavan

True, yes, rmap_walk may block, so it the try_to_unmap calls should be
outside the loop. The easiest thing to do there is to add all mapped
pages to a separate list, walk that list outside of the rcu lock for
i_mapping, and add all pages that could be unmapped to the return
list.

- Frank