linux-kernel - Re: [PATCH 3/8] swap: don't add ITER_BVEC flag to direct

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Date:	Mon, 15 Dec 2014 07:57:07 -0800
From:	Omar Sandoval <osandov@...ndov.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Christoph Hellwig <hch@...radead.org>,
	David Sterba <dsterba@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] swap: don't add ITER_BVEC flag to direct_IO rw

On Mon, Dec 15, 2014 at 06:16:02AM +0000, Al Viro wrote:
> On Sun, Dec 14, 2014 at 09:26:57PM -0800, Omar Sandoval wrote:
> > The rw argument to direct_IO has some ill-defined semantics. Some
> > filesystems (e.g., ext4, FAT) decide whether they're doing a write with
> > rw == WRITE, but others (e.g., XFS) check rw & WRITE. Let's set a good
> > example in the swap file code and say ITER_BVEC belongs in
> > iov_iter->flags but not in rw. This caters to the least common
> > denominator and avoids a sweeping change of every direct_IO
> > implementation for now.
> 
> Frankly, this is bogus.  If anything, let's just kill the first argument
> completely - ->direct_IO() can always pick it from iter->type.
> 
> As for catering to the least common denominator...  To hell with the lowest
> common denominator.  How many instances of ->direct_IO() do we have, anyway?
> 24 in the mainline (and we don't give a flying fuck for out-of-tree code, as
> a matter of policy).  Moreover, several are of "do nothing" variety.
> 
> FWIW, 'rw' is a mess.  We used to have this:
> 	READ: O_DIRECT read
> 	WRITE: O_DIRECT write
> 	KERNEL_WRITE: swapout
> 
> These days KERNEL_WRITE got replaced with ITER_BVEC | WRITE.  The thing is,
> we have a bunch of places where we explicitly checked for being _equal_ to
> WRITE.  I.e. the checks that gave a negative on swapouts.  I suspect that most
> of them are wrong and should trigger on all writes, including swapouts, but
> I really didn't want to dig into that pile of fun back then.  That's the
> main reason why 'rw' argument has survived at all...
>
In that case, I'll take a stab at nuking rw. I'm almost certain that
some of these are completely wrong (for example, of the form
if (rw == WRITE) do_write(); else do_read();). This isn't an immediate
problem for swap files on BTRFS, as __blockdev_direct_IO does a bitwise
test, so I think I'll split it out into its own series.

Thanks,
-- 
Omar
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