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

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Message-ID: <20170301153857.GC30631@infradead.org>
Date:   Wed, 1 Mar 2017 07:38:57 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Goldwyn Rodrigues <rgoldwyn@...e.de>, jack@...e.com,
        hch@...radead.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 3/8] nowait aio: return if direct write will trigger
 writeback

On Tue, Feb 28, 2017 at 07:46:06PM -0800, Matthew Wilcox wrote:
> Ugh, this is pretty inefficient.  If that's all you want to know, then
> using the radix tree directly will be far more efficient than spinning
> up all the pagevec machinery only to discard the pages found.
> 
> But what's going to kick these pages out of cache?  Shouldn't we rather
> find the pages, kick them out if clean, start writeback if not, and *then*
> return -EAGAIN?
> 
> So maybe we want to spin up the pagevec machinery after all so we can
> do that extra work?

As pointed out in the last round of these patches I think we really
need to pass a flags argument to filemap_write_and_wait_range to
communicate the non-blocking nature and only return -EAGAIN if we'd
block.  As a bonus that can indeed start to kick the pages out.