Re: [RFC 3/4] mm/zswap: add support for large folio zswapin

[Barry Song <21cnbao@gmail.com>]

On Mon, Oct 21, 2024 at 11:44 PM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 21/10/2024 06:49, Barry Song wrote:
> > On Fri, Oct 18, 2024 at 11:50 PM Usama Arif <usamaarif642@gmail.com> wrote:
> >>
> >> At time of folio allocation, alloc_swap_folio checks if the entire
> >> folio is in zswap to determine folio order.
> >> During swap_read_folio, zswap_load will check if the entire folio
> >> is in zswap, and if it is, it will iterate through the pages in
> >> folio and decompress them.
> >> This will mean the benefits of large folios (fewer page faults, batched
> >> PTE and rmap manipulation, reduced lru list, TLB coalescing (for arm64
> >> and amd) are not lost at swap out when using zswap.
> >> This patch does not add support for hybrid backends (i.e. folios
> >> partly present swap and zswap).
> >>
> >> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> >> ---
> >>  mm/memory.c | 13 +++-------
> >>  mm/zswap.c  | 68 ++++++++++++++++++++++++-----------------------------
> >>  2 files changed, 34 insertions(+), 47 deletions(-)
> >>
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index 49d243131169..75f7b9f5fb32 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -4077,13 +4077,14 @@ static bool can_swapin_thp(struct vm_fault *vmf, pte_t *ptep, int nr_pages)
> >>
> >>         /*
> >>          * swap_read_folio() can't handle the case a large folio is hybridly
> >> -        * from different backends. And they are likely corner cases. Similar
> >> -        * things might be added once zswap support large folios.
> >> +        * from different backends. And they are likely corner cases.
> >>          */
> >>         if (unlikely(swap_zeromap_batch(entry, nr_pages, NULL) != nr_pages))
> >>                 return false;
> >>         if (unlikely(non_swapcache_batch(entry, nr_pages) != nr_pages))
> >>                 return false;
> >> +       if (unlikely(!zswap_present_test(entry, nr_pages)))
> >> +               return false;
> >>
> >>         return true;
> >>  }
> >> @@ -4130,14 +4131,6 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
> >>         if (unlikely(userfaultfd_armed(vma)))
> >>                 goto fallback;
> >>
> >> -       /*
> >> -        * A large swapped out folio could be partially or fully in zswap. We
> >> -        * lack handling for such cases, so fallback to swapping in order-0
> >> -        * folio.
> >> -        */
> >> -       if (!zswap_never_enabled())
> >> -               goto fallback;
> >> -
> >>         entry = pte_to_swp_entry(vmf->orig_pte);
> >>         /*
> >>          * Get a list of all the (large) orders below PMD_ORDER that are enabled
> >> diff --git a/mm/zswap.c b/mm/zswap.c
> >> index 9cc91ae31116..a5aa86c24060 100644
> >> --- a/mm/zswap.c
> >> +++ b/mm/zswap.c
> >> @@ -1624,59 +1624,53 @@ bool zswap_present_test(swp_entry_t swp, int nr_pages)
> >>
> >>  bool zswap_load(struct folio *folio)
> >>  {
> >> +       int nr_pages = folio_nr_pages(folio);
> >>         swp_entry_t swp = folio->swap;
> >> +       unsigned int type = swp_type(swp);
> >>         pgoff_t offset = swp_offset(swp);
> >>         bool swapcache = folio_test_swapcache(folio);
> >> -       struct xarray *tree = swap_zswap_tree(swp);
> >> +       struct xarray *tree;
> >>         struct zswap_entry *entry;
> >> +       int i;
> >>
> >>         VM_WARN_ON_ONCE(!folio_test_locked(folio));
> >>
> >>         if (zswap_never_enabled())
> >>                 return false;
> >>
> >> -       /*
> >> -        * Large folios should not be swapped in while zswap is being used, as
> >> -        * they are not properly handled. Zswap does not properly load large
> >> -        * folios, and a large folio may only be partially in zswap.
> >> -        *
> >> -        * Return true without marking the folio uptodate so that an IO error is
> >> -        * emitted (e.g. do_swap_page() will sigbus).
> >> -        */
> >> -       if (WARN_ON_ONCE(folio_test_large(folio)))
> >> -               return true;
> >> -
> >> -       /*
> >> -        * When reading into the swapcache, invalidate our entry. The
> >> -        * swapcache can be the authoritative owner of the page and
> >> -        * its mappings, and the pressure that results from having two
> >> -        * in-memory copies outweighs any benefits of caching the
> >> -        * compression work.
> >> -        *
> >> -        * (Most swapins go through the swapcache. The notable
> >> -        * exception is the singleton fault on SWP_SYNCHRONOUS_IO
> >> -        * files, which reads into a private page and may free it if
> >> -        * the fault fails. We remain the primary owner of the entry.)
> >> -        */
> >> -       if (swapcache)
> >> -               entry = xa_erase(tree, offset);
> >> -       else
> >> -               entry = xa_load(tree, offset);
> >> -
> >> -       if (!entry)
> >> +       if (!zswap_present_test(folio->swap, nr_pages))
> >>                 return false;
> >
> > Hi Usama,
> >
> > Is there any chance that zswap_present_test() returns true
> > in do_swap_page() but false in zswap_load()? If that’s
> > possible, could we be missing something? For example,
> > could it be that zswap has been partially released (with
> > part of it still present) during an mTHP swap-in?
> >
> > If this happens with an mTHP, my understanding is that
> > we shouldn't proceed with reading corrupted data from the
> > disk backend.
> >
>
> If its not swapcache, the zswap entry is not deleted so I think
> it should be ok?
>
> We can check over here if the entire folio is in zswap,
> and if not, return true without marking the folio uptodate
> to give an error.

We have swapcache_prepare() called in do_swap_page(), which should
have protected these entries from being partially freed by other processes
(for example, if someone falls back to small folios for the same address).
Therefore, I believe that zswap_present_test() cannot be false for mTHP in
the current case where only synchronous I/O is supported.

the below might help detect the bug?

if (!zswap_present_test(folio->swap, nr_pages)) {
     if (WARN_ON_ONCE(nr_pages > 1))
                return true;
     return false;
}

the code seems quite ugly :-) do we have some way to unify the code
for large and small folios?

not quite sure about shmem though....

>
>
> >>
> >> -       zswap_decompress(entry, &folio->page);
> >> +       for (i = 0; i < nr_pages; ++i) {
> >> +               tree = swap_zswap_tree(swp_entry(type, offset + i));
> >> +               /*
> >> +                * When reading into the swapcache, invalidate our entry. The
> >> +                * swapcache can be the authoritative owner of the page and
> >> +                * its mappings, and the pressure that results from having two
> >> +                * in-memory copies outweighs any benefits of caching the
> >> +                * compression work.
> >> +                *
> >> +                * (Swapins with swap count > 1 go through the swapcache.
> >> +                * For swap count == 1, the swapcache is skipped and we
> >> +                * remain the primary owner of the entry.)
> >> +                */
> >> +               if (swapcache)
> >> +                       entry = xa_erase(tree, offset + i);
> >> +               else
> >> +                       entry = xa_load(tree, offset + i);
> >>
> >> -       count_vm_event(ZSWPIN);
> >> -       if (entry->objcg)
> >> -               count_objcg_events(entry->objcg, ZSWPIN, 1);
> >> +               zswap_decompress(entry, folio_page(folio, i));
> >>
> >> -       if (swapcache) {
> >> -               zswap_entry_free(entry);
> >> -               folio_mark_dirty(folio);
> >> +               if (entry->objcg)
> >> +                       count_objcg_events(entry->objcg, ZSWPIN, 1);
> >> +               if (swapcache)
> >> +                       zswap_entry_free(entry);
> >>         }
> >>
> >> +       count_vm_events(ZSWPIN, nr_pages);
> >> +       if (swapcache)
> >> +               folio_mark_dirty(folio);
> >> +
> >>         folio_mark_uptodate(folio);
> >>         return true;
> >>  }
> >> --
> >> 2.43.5
> >>
> >

Thanks
barry