linux-kernel - Re: [PATCH 0/3] block: loop: switch to VFS ITER

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Message-ID: <20150331222247.GK29656@ZenIV.linux.org.uk>
Date:	Tue, 31 Mar 2015 23:22:48 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Christoph Hellwig <hch@....de>
Cc:	Ming Lei <ming.lei@...onical.com>, Jens Axboe <axboe@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Maxim Patlasov <mpatlasov@...allels.com>
Subject: Re: [PATCH 0/3] block: loop: switch to VFS ITER_BVEC

On Wed, Mar 25, 2015 at 04:27:48PM +0100, Christoph Hellwig wrote:
> On Wed, Mar 25, 2015 at 03:23:48PM +0800, Ming Lei wrote:
> > > -       mm_segment_t old_fs = get_fs();
> > > +
> > > +       iov_iter_bvec(&i, ITER_BVEC, bvec, 1, bvec->bv_len);
> > >
> > >         file_start_write(file);
> > > -       set_fs(get_ds());
> > > -       bw = file->f_op->write(file, buf, len, &pos);
> > > -       set_fs(old_fs);
> > > +       bw = vfs_iter_write(file, &i, ppos);
> > 
> > This patch moves to support ->read_iter/->write_iter only, which
> > might cause regression for backing file without defining read/write
> > iter callback.
> 
> ->read_iter/->write_iter is the main fs I/O path - by the time this is
> ready ->aio_read/->aio_write should be gone.

Umm...  FWIW, it's not about ->aio_write - that conversion is practically
complete; nothing with ->aio_write != NULL is plausible for backing store
of /dev/loop anyway.  Regression in question is about the files that
have ->write(), but no ->write_iter().  Most of that won't be usable as
backing store, but there are some exceptions:
	* ncpfs regular files.  Ought to be switched to {read,write}_iter -
it's a pretty mechanical work, I'll do that.  Only marginally useful,
but what the hell - why not do it right?
	* coda.  Ought to switch to ->..._iter() and demand _its_ backing
store to have those.  Trivial, will do.
	* 9p.  Cthulhu-awful mess.  The trouble is, we'd need to support
ITER_BVEC ones down in p9_client_write().  I'm actually trying to do that
right now; if it works, this regression will become a non-issue.
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