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

Usama Arif
2024-10-21T20:57:20Z

On 21/10/2024 21:28, Barry Song wrote:
> On Tue, Oct 22, 2024 at 1:21 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>
>>
>>
>> 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.
>
> my point is that zswap_present_test() can't differentiate
> 1. the *whole* folio is not in zswap
> 2. the folio is *partially* not in zswap
>
> in case 2, returning false is wrong.
>

Agreed!

> And when nr_pages > 1, we have already confirmed earlier in
> do_swap_page() that zswap_present_test() is true. At this point,
> it must always be true; if it's false, it indicates a bug.
>

Yes agreed! I was thinking from just zswap_load perspective irrespective
of who calls it.
If someone adds large folio support to swapin_readahead, then I think the
above warn might be an issue.

But just with this patch series, doing what you suggested is correct. I
will add it in next revision. We can deal with it once swap count > 1,
starts supporting large folios.

>>
>>> 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
>>