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

Usama Arif
2024-10-21T12:21:07Z

On 21/10/2024 11:55, Barry Song wrote:
> 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;
> }
>

I think this isn't correct. If nr_pages > 1 and the entire folio is not in zswap,
it should still return false. So would need to check the whole folio if we want to
warn. But I think if we are sure the code is ok, it is an unnecessary check.

> 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....
>

If its shmem, and the swap_count goes to 1, I think its still ok? because
then the folio will be gotten from swap_cache_get_folio if it has already
been in swapcache.

>>
>>
>>>>
>>>> - 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