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

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Date:   Thu, 2 Mar 2017 11:38:45 +0100
From:   Jan Kara <jack@...e.cz>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>, jack@...e.com,
        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 Wed 01-03-17 07:38:57, Christoph Hellwig wrote:
> 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.

Aren't flags to filemap_write_and_wait_range() unnecessary complication?
Realistically, most users wanting performance from AIO DIO so badly that
they bother with this API won't have any pages to write / evict. If they do
by some bad accident, they can fall back to standard "blocking" AIO DIO.
So I don't see much value in teaching filemap_write_and_wait_range() about
a non-blocking mode...

								Honza

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