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

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Message-ID: <9a5474b6-aaac-4567-9405-351d6755f947@kernel.dk>
Date: Tue, 12 Nov 2024 07:09:02 -0700
From: Jens Axboe <axboe@...nel.dk>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, hannes@...xchg.org,
 clm@...a.com, linux-kernel@...r.kernel.org, willy@...radead.org,
 linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
 linux-xfs@...r.kernel.org
Subject: Re: [PATCH 09/16] mm/filemap: drop uncached pages when writeback
 completes

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@...nel.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