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From: Al Viro <viro@zeniv.linux.org.uk>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>,
	Ilya Dryomov <idryomov@gmail.com>, Jens Axboe <axboe@kernel.dk>,
	Jeff Layton <jlayton@kernel.org>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-block@vger.kernel.org, ceph-devel@vger.kernel.org,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/5] bio: Direct IO: convert to pin_user_pages_fast()
Date: Tue, 25 Aug 2020 03:07:00 +0100	[thread overview]
Message-ID: <20200825020700.GV1236603@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200825015428.GU1236603@ZenIV.linux.org.uk>

On Tue, Aug 25, 2020 at 02:54:28AM +0100, Al Viro wrote:
> On Fri, Aug 21, 2020 at 09:20:54PM -0700, John Hubbard wrote:
> 
> > Direct IO behavior:
> > 
> >     ITER_IOVEC:
> >         pin_user_pages_fast();
> >         break;
> > 
> >     ITER_KVEC:    // already elevated page refcount, leave alone
> >     ITER_BVEC:    // already elevated page refcount, leave alone
> >     ITER_PIPE:    // just, no :)
> 
> Why?  What's wrong with splice to O_DIRECT file?

Sorry - s/to/from/, obviously.

To spell it out: consider generic_file_splice_read() behaviour when
the source had been opened with O_DIRECT; you will get a call of
->read_iter() into ITER_PIPE destination.  And it bloody well
will hit iov_iter_get_pages() on common filesystems, to pick the
pages we want to read into.

So... what's wrong with having that "pin" primitive making sure
the pages are there and referenced by the pipe?

  reply	other threads:[~2020-08-25  2:07 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-22  4:20 John Hubbard
2020-08-22  4:20 ` [PATCH 1/5] iov_iter: introduce iov_iter_pin_user_pages*() routines John Hubbard
2020-08-22  4:20 ` [PATCH 2/5] mm/gup: introduce pin_user_page() John Hubbard
2020-08-22  4:20 ` [PATCH 3/5] bio: convert get_user_pages_fast() --> pin_user_pages_fast() John Hubbard
2020-08-22  4:20 ` [PATCH 4/5] bio: introduce BIO_FOLL_PIN flag John Hubbard
2020-08-23  6:25   ` Christoph Hellwig
2020-08-23  6:57     ` John Hubbard
2020-08-24  7:36       ` John Hubbard
2020-08-24  9:20     ` Jens Axboe
2020-08-24 14:42       ` Christoph Hellwig
2020-08-22  4:20 ` [PATCH 5/5] fs/ceph: use pipe_get_pages_alloc() for pipe John Hubbard
2020-08-24 10:53   ` Jeff Layton
2020-08-24 17:54     ` John Hubbard
2020-08-24 18:47       ` Jeff Layton
2020-08-24 18:54         ` Matthew Wilcox
2020-08-24 19:02           ` John Hubbard
2020-08-25  1:20           ` [PATCH v2] " John Hubbard
2020-08-25 16:22             ` Jeff Layton
2020-08-25  1:54 ` [PATCH 0/5] bio: Direct IO: convert to pin_user_pages_fast() Al Viro
2020-08-25  2:07   ` Al Viro [this message]
2020-08-25  2:13     ` John Hubbard
2020-08-25  2:07   ` John Hubbard
2020-08-25  2:22     ` Al Viro
2020-08-25  2:27       ` John Hubbard

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