Re: [PATCH 1/3] mm: provide filemap_range_needs_writeback() helper

[Jan Kara <jack@suse.cz>]

On Wed 24-02-21 09:44:53, Jens Axboe wrote:
> For O_DIRECT reads/writes, we check if we need to issue a call to
> filemap_write_and_wait_range() to issue and/or wait for writeback for any
> page in the given range. The existing mechanism just checks for a page in
> the range, which is suboptimal for IOCB_NOWAIT as we'll fallback to the
> slow path (and needing retry) if there's just a clean page cache page in
> the range.
> 
> Provide filemap_range_needs_writeback() which tries a little harder to
> check if we actually need to issue and/or wait for writeback in the
> range.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

Looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  include/linux/fs.h |  2 ++
>  mm/filemap.c       | 43 +++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 45 insertions(+)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 6d8b1e7337e4..4925275e6365 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2633,6 +2633,8 @@ static inline int filemap_fdatawait(struct address_space *mapping)
>  
>  extern bool filemap_range_has_page(struct address_space *, loff_t lstart,
>  				  loff_t lend);
> +extern bool filemap_range_needs_writeback(struct address_space *,
> +					  loff_t lstart, loff_t lend);
>  extern int filemap_write_and_wait_range(struct address_space *mapping,
>  				        loff_t lstart, loff_t lend);
>  extern int __filemap_fdatawrite_range(struct address_space *mapping,
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 6ff2a3fb0dc7..13338f877677 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -635,6 +635,49 @@ static bool mapping_needs_writeback(struct address_space *mapping)
>  	return mapping->nrpages;
>  }
>  
> +/**
> + * filemap_range_needs_writeback - check if range potentially needs writeback
> + * @mapping:           address space within which to check
> + * @start_byte:        offset in bytes where the range starts
> + * @end_byte:          offset in bytes where the range ends (inclusive)
> + *
> + * Find at least one page in the range supplied, usually used to check if
> + * direct writing in this range will trigger a writeback. Used by O_DIRECT
> + * read/write with IOCB_NOWAIT, to see if the caller needs to do
> + * filemap_write_and_wait_range() before proceeding.
> + *
> + * Return: %true if the caller should do filemap_write_and_wait_range() before
> + * doing O_DIRECT to a page in this range, %false otherwise.
> + */
> +bool filemap_range_needs_writeback(struct address_space *mapping,
> +				   loff_t start_byte, loff_t end_byte)
> +{
> +	XA_STATE(xas, &mapping->i_pages, start_byte >> PAGE_SHIFT);
> +	pgoff_t max = end_byte >> PAGE_SHIFT;
> +	struct page *page;
> +
> +	if (!mapping_needs_writeback(mapping))
> +		return false;
> +	if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY) &&
> +	    !mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
> +		return false;
> +	if (end_byte < start_byte)
> +		return false;
> +
> +	rcu_read_lock();
> +	xas_for_each(&xas, page, max) {
> +		if (xas_retry(&xas, page))
> +			continue;
> +		if (xa_is_value(page))
> +			continue;
> +		if (PageDirty(page) || PageLocked(page) || PageWriteback(page))
> +			break;
> +	}
> +	rcu_read_unlock();
> +	return page != NULL;
> +}
> +EXPORT_SYMBOL_GPL(filemap_range_needs_writeback);
> +
>  /**
>   * filemap_write_and_wait_range - write out & wait on a file range
>   * @mapping:	the address_space for the pages
> -- 
> 2.30.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR