linux-kernel - Re: [PATCH v2 04/12] mm/filemap: Add mapping_seek_hole

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Date:   Tue, 29 Sep 2020 10:46:53 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Dave Chinner <dchinner@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 04/12] mm/filemap: Add mapping_seek_hole_data

On Mon 14-09-20 14:00:34, Matthew Wilcox (Oracle) wrote:
> Rewrite shmem_seek_hole_data() and move it to filemap.c.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
...
> +/**
> + * mapping_seek_hole_data - Seek for SEEK_DATA / SEEK_HOLE in the page cache.
> + * @mapping: Address space to search.
> + * @start: First byte to consider.
> + * @end: Limit of search (exclusive).
> + * @whence: Either SEEK_HOLE or SEEK_DATA.
> + *
> + * If the page cache knows which blocks contain holes and which blocks
> + * contain data, your filesystem can use this function to implement
> + * SEEK_HOLE and SEEK_DATA.  This is useful for filesystems which are
> + * entirely memory-based such as tmpfs, and filesystems which support
> + * unwritten extents.
> + *
> + * Return: The requested offset on successs, or -ENXIO if @whence specifies
> + * SEEK_DATA and there is no data after @start.  There is an implicit hole
> + * after @end - 1, so SEEK_HOLE returns @end if all the bytes between @start
> + * and @end contain data.
> + */
> +loff_t mapping_seek_hole_data(struct address_space *mapping, loff_t start,
> +		loff_t end, int whence)
> +{
> +	XA_STATE(xas, &mapping->i_pages, start >> PAGE_SHIFT);
> +	pgoff_t max = (end - 1) / PAGE_SIZE;
> +	bool seek_data = (whence == SEEK_DATA);
> +	struct page *page;
> +
> +	if (end <= start)
> +		return -ENXIO;
> +
> +	rcu_read_lock();
> +	while ((page = xas_find_get_entry(&xas, max, XA_PRESENT))) {
> +		loff_t pos = xas.xa_index * PAGE_SIZE;

OK, but for ordinary filesystems this could be problematic because of
exceptional entries?

Also for shmem you've dropped the PageUptodate check which I'm not sure is
safe?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR