linux-kernel - Re: [PATCH 5/6] NFS: direct-io: convert to FOLL

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Date:   Mon, 29 Aug 2022 18:08:08 +0200
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>, Jan Kara <jack@...e.cz>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages

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@...e.com>
SUSE Labs, CR