linux-kernel - Re: [PATCH v3 04/12] mm/filemap: Add mapping_seek_hole

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Message-ID: <20201027185809.GB15201@infradead.org>
Date:   Tue, 27 Oct 2020 18:58:09 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Dave Chinner <dchinner@...hat.com>,
        linux-kernel@...r.kernel.org,
        William Kucharski <william.kucharski@...cle.com>
Subject: Re: [PATCH v3 04/12] mm/filemap: Add mapping_seek_hole_data

> +/**
> + * 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.
> + */

This seems to just lift the tmpfs one to common code.  If it really
is supposed to be generic it should be able to replace
page_cache_seek_hole_data as well.  So I don't think moving this without
removing the other common one is an all that good idea.