Re: [PATCH 09/16] mm/filemap: drop uncached pages when writeback completes

Jens Axboe
2024-11-12T14:09:05Z

On 11/12/24 2:31 AM, Kirill A. Shutemov wrote:
> On Mon, Nov 11, 2024 at 04:37:36PM -0700, Jens Axboe wrote:
>> If the folio is marked as uncached, drop pages when writeback completes.
>> Intended to be used with RWF_UNCACHED, to avoid needing sync writes for
>> uncached IO.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>> mm/filemap.c | 28 ++++++++++++++++++++++++++++
>> 1 file changed, 28 insertions(+)
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 3d0614ea5f59..40debe742abe 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -1600,6 +1600,27 @@ int folio_wait_private_2_killable(struct folio *folio)
>> }
>> EXPORT_SYMBOL(folio_wait_private_2_killable);
>>
>> +/*
>> + * If folio was marked as uncached, then pages should be dropped when writeback
>> + * completes. Do that now. If we fail, it's likely because of a big folio -
>> + * just reset uncached for that case and latter completions should invalidate.
>> + */
>> +static void folio_end_uncached(struct folio *folio)
>> +{
>> + /*
>> + * Hitting !in_task() should not happen off RWF_UNCACHED writeback, but
>> + * can happen if normal writeback just happens to find dirty folios
>> + * that were created as part of uncached writeback, and that writeback
>> + * would otherwise not need non-IRQ handling. Just skip the
>> + * invalidation in that case.
>> + */
>> + if (in_task() && folio_trylock(folio)) {
>> + if (folio->mapping)
>> + folio_unmap_invalidate(folio->mapping, folio, 0);
>> + folio_unlock(folio);
>> + }
>> +}
>> +
>> /**
>> * folio_end_writeback - End writeback against a folio.
>> * @folio: The folio.
>> @@ -1610,6 +1631,8 @@ EXPORT_SYMBOL(folio_wait_private_2_killable);
>> */
>> void folio_end_writeback(struct folio *folio)
>> {
>> + bool folio_uncached = false;
>> +
>> VM_BUG_ON_FOLIO(!folio_test_writeback(folio), folio);
>>
>> /*
>> @@ -1631,9 +1654,14 @@ void folio_end_writeback(struct folio *folio)
>> * reused before the folio_wake_bit().
>> */
>> folio_get(folio);
>> + if (folio_test_uncached(folio) && folio_test_clear_uncached(folio))
>> + folio_uncached = true;
>
> Hm? Maybe
>
> folio_uncached = folio_test_clear_uncached(folio);
>
> ?

It's done that way to avoid a RMW for the (for now, at least) common
case of not seeing cached folios. For that case, you can get by with a
cheap test_bit, for the cached case you pay the full price of the
test_clear.

Previous versions just had the test_clear, happy to just go back or add
a comment, whatever is preferred.

--
Jens Axboe