Re: [PATCH v2] mm: zswap: handle incorrect attempts to load of large folios

[Mika Penttilä]

On 6/10/24 20:35, Yosry Ahmed wrote:
> On Fri, Jun 7, 2024 at 9:08 PM Mika Penttilä <mpenttil@xxxxxxxxxx> wrote:
>> On 6/8/24 05:36, Yosry Ahmed wrote:
>>> diff --git a/mm/zswap.c b/mm/zswap.c
>>> index b9b35ef86d9be..ebb878d3e7865 100644
>>> --- a/mm/zswap.c
>>> +++ b/mm/zswap.c
>>> @@ -1557,6 +1557,26 @@ bool zswap_load(struct folio *folio)
>>>
>>>       VM_WARN_ON_ONCE(!folio_test_locked(folio));
>>>
>>> +     /*
>>> +      * 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.
>>> +      *
>>> +      * If any of the subpages are in zswap, reading from disk would result
>>> +      * in data corruption, so return true without marking the folio uptodate
>>> +      * so that an IO error is emitted (e.g. do_swap_page() will sigfault).
>>> +      *
>>> +      * Otherwise, return false and read the folio from disk.
>>> +      */
>>> +     if (folio_test_large(folio)) {
>>> +             if (xa_find(tree, &offset,
>>> +                         offset + folio_nr_pages(folio) - 1, XA_PRESENT)) {
>>> +                     WARN_ON_ONCE(1);
>>> +                     return true;
>>> +             }
>> How does that work? Should it be xa_find_after() to not always find
>> current entry?
> By "current entry" I believe you mean the entry corresponding to
> "offset" (i.e. the first subpage of the folio). At this point, we
> haven't checked if that offset has a corresponding entry in zswap or
> not. It may be on disk, or zwap may be disabled.

Okay you test if there's any matching offset in zswap for the folio.


>> And does it still mean those subsequent entries map to same folio?
> If I understand correctly, a folio in the swapcache has contiguous
> swap offsets for its subpages. So I am assuming that the large folio
> swapin case will adhere to that (i.e. we only swapin a large folio if
> the swap offsets are contiguous). Did I misunderstand something here?

Yes I think that is fair assumption for now. But also saw your v3 which
doesn't depend on this.


>
>>
>> --Mika
>>
>>