From: Jan Kara <jack@suse.cz>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
Andrew Morton <akpm@linux-foundation.org>,
Jens Axboe <axboe@kernel.dk>, Miklos Szeredi <miklos@szeredi.hu>,
Christoph Hellwig <hch@infradead.org>,
"Darrick J . Wong" <djwong@kernel.org>,
Trond Myklebust <trond.myklebust@hammerspace.com>,
Anna Schumaker <anna@kernel.org>, Jan Kara <jack@suse.cz>,
Logan Gunthorpe <logang@deltatee.com>,
linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org,
linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages
Date: Mon, 29 Aug 2022 18:08:08 +0200 [thread overview]
Message-ID: <20220829160808.rwkkiuelipr3huxk@quack3> (raw)
In-Reply-To: <217b4a17-1355-06c5-291e-7980c0d3cea6@nvidia.com>
On Sun 28-08-22 21:59:49, John Hubbard wrote:
> On 8/27/22 17:39, Al Viro wrote:
> > On Sun, Aug 28, 2022 at 01:38:57AM +0100, Al Viro wrote:
> >> On Sat, Aug 27, 2022 at 04:55:18PM -0700, John Hubbard wrote:
> >>> On 8/27/22 15:48, Al Viro wrote:
> >>>> On Sat, Aug 27, 2022 at 01:36:06AM -0700, John Hubbard wrote:
> >>>>> Convert the NFS Direct IO layer to use pin_user_pages_fast() and
> >>>>> unpin_user_page(), instead of get_user_pages_fast() and put_page().
> >>>>
> >>>> Again, this stuff can be hit with ITER_BVEC iterators
> >>>>
> >>>>> - result = iov_iter_get_pages_alloc2(iter, &pagevec,
> >>>>> + result = dio_w_iov_iter_pin_pages_alloc(iter, &pagevec,
> >>>>> rsize, &pgbase);
> >>>>
> >>>> and this will break on those.
> >>>
> >>> If anyone has an example handy, of a user space program that leads
> >>> to this situation (O_DIRECT with ITER_BVEC), it would really help
> >>> me reach enlightenment a lot quicker in this area. :)
> >>
> >> Er... splice(2) to O_DIRECT-opened file on e.g. ext4? Or
> >> sendfile(2) to the same, for that matter...
> >
> > s/ext4/nfs/ to hit this particular codepath, obviously.
>
> OK, I have a solution to this that's pretty easy:
>
> 1) Get rid of the user_backed_iter(i) check in
> dio_w_iov_iter_pin_pages() and dio_w_iov_iter_pin_pages_alloc(), and
>
> 2) At the call sites, match up the unpin calls appropriately.
>
> ...and apply a similar fix for the fuse conversion patch.
>
> However, the core block/bio conversion in patch 4 still does depend upon
> a key assumption, which I got from a 2019 email discussion with
> Christoph Hellwig and others here [1], which says:
>
> "All pages released by bio_release_pages should come from
> get_get_user_pages...".
>
> I really hope that still holds true. Otherwise this whole thing is in
> trouble.
>
> [1] https://lore.kernel.org/kvm/20190724053053.GA18330@infradead.org/
Well as far as I've checked that discussion, Christoph was aware of pipe
pages etc. (i.e., bvecs) entering direct IO code. But he had some patches
[2] which enabled GUP to work for bvecs as well (using the kernel mapping
under the hood AFAICT from a quick glance at the series). I suppose we
could also handle this in __iov_iter_get_pages_alloc() by grabbing pin
reference instead of plain get_page() for the case of bvec iter. That way
we should have only pinned pages in bio_release_pages() even for the bvec
case.
[2] http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2022-08-29 16:08 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-27 8:36 [PATCH 0/6] convert most filesystems to pin_user_pages_fast() John Hubbard
2022-08-27 8:36 ` [PATCH 1/6] mm/gup: introduce pin_user_page() John Hubbard
2022-08-29 12:07 ` David Hildenbrand
2022-08-29 19:33 ` John Hubbard
2022-08-30 12:17 ` David Hildenbrand
2022-08-30 21:42 ` John Hubbard
2022-08-31 0:06 ` John Hubbard
2022-08-27 8:36 ` [PATCH 2/6] block: add dio_w_*() wrappers for pin, unpin user pages John Hubbard
2022-08-27 22:27 ` Andrew Morton
2022-08-27 23:59 ` John Hubbard
2022-08-28 0:12 ` Andrew Morton
2022-08-28 0:31 ` John Hubbard
2022-08-28 1:07 ` John Hubbard
2022-08-27 8:36 ` [PATCH 3/6] iov_iter: new iov_iter_pin_pages*() routines John Hubbard
2022-08-27 22:46 ` Al Viro
2022-08-27 22:48 ` John Hubbard
2022-08-27 8:36 ` [PATCH 4/6] block, bio, fs: convert most filesystems to pin_user_pages_fast() John Hubbard
2022-08-27 8:36 ` [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages John Hubbard
2022-08-27 22:48 ` Al Viro
2022-08-27 23:55 ` John Hubbard
2022-08-28 0:38 ` Al Viro
2022-08-28 0:39 ` Al Viro
2022-08-28 0:46 ` John Hubbard
2022-08-29 4:59 ` John Hubbard
2022-08-29 16:08 ` Jan Kara [this message]
2022-08-29 19:59 ` John Hubbard
2022-08-31 9:43 ` Jan Kara
2022-08-31 18:02 ` John Hubbard
2022-09-01 0:38 ` Al Viro
2022-09-01 9:06 ` Jan Kara
2022-08-27 8:36 ` [PATCH 6/6] fuse: convert direct IO paths to use FOLL_PIN John Hubbard
-- strict thread matches above, loose matches on Subject: below --
2022-02-27 9:34 [PATCH 0/6] block, fs: convert most Direct IO cases to FOLL_PIN jhubbard.send.patches
2022-02-27 9:34 ` [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages jhubbard.send.patches
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