linux-kernel - Re: [PATCH v4 1/5] fs: kiocb: introduce IOCB_DONT_DIRTY

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Date:	Mon, 8 Jun 2015 23:29:43 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Jens Axboe <axboe@...nel.dk>, Zach Brown <zab@...bo.net>,
	Maxim Patlasov <mpatlasov@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Tejun Heo <tj@...nel.org>, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v4 1/5] fs: kiocb: introduce IOCB_DONT_DIRTY_PAGE flag
 for direct IO

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.
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