linux-ext4 - Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

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Message-ID: <20170316132052.GG4033@bombadil.infradead.org>
Date:   Thu, 16 Mar 2017 06:20:52 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Goldwyn Rodrigues <rgoldwyn@...e.de>
Cc:     linux-fsdevel@...r.kernel.org, jack@...e.com, hch@...radead.org,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        sagi@...mberg.me, avi@...lladb.com, axboe@...nel.dk,
        linux-api@...r.kernel.org, Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 3/8] nowait aio: return if direct write will trigger
 writeback

On Wed, Mar 15, 2017 at 04:51:02PM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@...e.com>
> 
> Find out if the write will trigger a wait due to writeback. If yes,
> return -EAGAIN.
> 
> This introduces a new function filemap_range_has_page() which
> returns true if the file's mapping has a page within the range
> mentioned.
> 
> Return -EINVAL for buffered AIO: there are multiple causes of
> delay such as page locks, dirty throttling logic, page loading
> from disk etc. which cannot be taken care of.

Also, this patch only touches the write path; we have a similar call to
write_and_wait_range() in generic_file_read_iter().

Actually, why do we even have that?  Why can't we satisfy an O_DIRECT
read from the cache?