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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20220831094349.boln4jjajkdtykx3@quack3>
Date:   Wed, 31 Aug 2022 11:43:49 +0200
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jan Kara <jack@...e.cz>, 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>,
        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 Mon 29-08-22 12:59:26, John Hubbard wrote:
> On 8/29/22 09:08, Jan Kara wrote:
> >> 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.
> 
> OK, thanks, that looks viable. So, that approach assumes that the
> remaining two cases in __iov_iter_get_pages_alloc() will never end up
> being released via bio_release_pages():
> 
>     iov_iter_is_pipe(i)
>     iov_iter_is_xarray(i)
> 
> I'm actually a little worried about ITER_XARRAY, which is a recent addition.
> It seems to be used in ways that are similar to ITER_BVEC, and cephfs is
> using it. It's probably OK for now, for this series, which doesn't yet
> convert cephfs.

So after looking into that a bit more, I think a clean approach would be to
provide iov_iter_pin_pages2() and iov_iter_pages_alloc2(), under the hood
in __iov_iter_get_pages_alloc() make sure we use pin_user_page() instead of
get_page() in all the cases (using this in pipe_get_pages() and
iter_xarray_get_pages() is easy) and then make all bio handling use the
pinning variants for iters. I think at least iov_iter_is_pipe() case needs
to be handled as well because as I wrote above, pipe pages can enter direct
IO code e.g. for splice(2).

Also I think that all iov_iter_get_pages2() (or the _alloc2 variant) users
actually do want the "pin page" semantics in the end (they are accessing
page contents) so eventually we should convert them all to
iov_iter_pin_pages2() and remove iov_iter_get_pages2() altogether. But this
will take some more conversion work with networking etc. so I'd start with
converting bios only.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR