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Date: Mon, 8 Jun 2015 23:29:43 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Ming Lei <ming.lei@canonical.com>
Cc: Christoph Hellwig <hch@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>, Tejun Heo <tj@kernel.org>,
        Dave Chinner <david@fromorbit.com>
Subject: Re: [PATCH v4 1/5] fs: kiocb: introduce IOCB_DONT_DIRTY_PAGE flag
 for direct IO
Message-ID: <20150609062943.GA32395@infradead.org>
References: <1432881338-898-1-git-send-email-ming.lei@canonical.com>
 <1432881338-898-2-git-send-email-ming.lei@canonical.com>
 <20150605150348.GA6835@infradead.org>
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On Sat, Jun 06, 2015 at 08:42:33AM +0800, Ming Lei wrote:
> Both ITER_KVEC and ITER_BVEC doesn't mean the pages are kernel
> page, for example of loop and swap. That is why this patch is more flexiable,
> and won't cause regression since the users may have different dirtying
> rules as you mentioned last time.
> 
> http://marc.info/?t=143093223200001&r=1&w=2

Again, we never dirty pages for the caller when doing general purpose
kernel I/O.  For loop we either don't need it (totally in-kernel block
I/O) or the caller takes care of it (direct I/O on the loop device).

The swap code currently only uses ->direct_IO and thus isn't affected
by this flag.  If we use it for write the low-level I/O code should not
dirty the page either.

So to repeat myself: for the current state of affairs adding a flag
that every sensible user has to set is a horrible interface.  If for
some unforseen reason we'll need the flag later on it should have
reverse polarity, and only be added when needed.