linux-kernel - Re: [PATCH v7 06/22] Replace XIP read and write with DAX I/O

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Date:	Sun, 13 Apr 2014 14:05:52 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 06/22] Replace XIP read and write with DAX I/O

On Wed, Apr 09, 2014 at 10:55:29PM +0200, Jan Kara wrote:
> > In addition to writing back dirty pages, filemap_write_and_wait_range()
> > will evict clean pages.  Unintuitive, I know, but it matches what the
> > direct I/O path does.  Plus, if we fall back to buffered I/O for holes
> > (see above), then this will do the right thing at that time.
>   Ugh, I'm pretty certain filemap_write_and_wait_range() doesn't evict
> anything ;). Direct IO path calls that function so that direct IO read
> after buffered write returns the written data. In that case we don't evict
> anything from page cache because direct IO read doesn't invalidate any
> information we have cached. Only direct IO write does that and for that we
> call invalidate_inode_pages2_range() after writing the pages. So I maintain
> that what you do doesn't make sense to me. You might need to do some
> invalidation of hole pages. But note that generic_file_direct_write() does
> that for you and even though that isn't serialized in any way with page
> faults which can instantiate the hole pages again, things should work out
> fine for you since that function also invalidates the range again after
> ->direct_IO callback is done. So AFAICT you don't have to do anything
> except writing some nice comment about this ;).

You're right.  I'm not sure what I got confused with there.  I don't
think there's a race I need to worry about ... even if another page gets
instantiated (consider one thread furiously loading from a hole as fast
as it can while another thread does a write), we'll shoot it down again.
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