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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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);

?

>  	if (__folio_end_writeback(folio))
>  		folio_wake_bit(folio, PG_writeback);
>  	acct_reclaim_writeback(folio);
> +
> +	if (folio_uncached)
> +		folio_end_uncached(folio);
>  	folio_put(folio);
>  }
>  EXPORT_SYMBOL(folio_end_writeback);
> -- 
> 2.45.2
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov