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

Barry Song
2024-10-21T10:55:55Z

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