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

Barry Song
2024-10-21T05:49:41Z

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.

>
> - 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