From: Dave Chinner <david@fromorbit.com>
To: David Laight <David.Laight@aculab.com>
Cc: 'David Howells' <dhowells@redhat.com>,
Al Viro <viro@zeniv.linux.org.uk>,
"torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
Ted Ts'o <tytso@mit.edu>,
Dave Hansen <dave.hansen@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
"willy@infradead.org" <willy@infradead.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"?
Date: Wed, 23 Jun 2021 08:20:44 +1000 [thread overview]
Message-ID: <20210622222044.GI2419729@dread.disaster.area> (raw)
In-Reply-To: <7a6d8c55749d46d09f6f6e27a99fde36@AcuMS.aculab.com>
On Tue, Jun 22, 2021 at 09:55:09PM +0000, David Laight wrote:
> From: David Howells
> > Sent: 22 June 2021 17:27
> >
> > Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > > On Tue, Jun 22, 2021 at 04:20:40PM +0100, David Howells wrote:
> > >
> > > > and wondering if the iov_iter_fault_in_readable() is actually effective.
> > > > Yes, it can make sure that the page we're intending to modify is dragged
> > > > into the pagecache and marked uptodate so that it can be read from, but is
> > > > it possible for the page to then get reclaimed before we get to
> > > > iov_iter_copy_from_user_atomic()? a_ops->write_begin() could potentially
> > > > take a long time, say if it has to go and get a lock/lease from a server.
> > >
> > > Yes, it is. So what? We'll just retry. You *can't* take faults while
> > > holding some pages locked; not without shitloads of deadlocks.
> >
> > In that case, can we amend the comment immediately above
> > iov_iter_fault_in_readable()?
> >
> > /*
> > * Bring in the user page that we will copy from _first_.
> > * Otherwise there's a nasty deadlock on copying from the
> > * same page as we're writing to, without it being marked
> > * up-to-date.
> > *
> > * Not only is this an optimisation, but it is also required
> > * to check that the address is actually valid, when atomic
> > * usercopies are used, below.
> > */
> > if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
> >
> > The first part suggests this is for deadlock avoidance. If that's not true,
> > then this should perhaps be changed.
>
> I'd say something like:
> /*
> * The actual copy_from_user() is done with a lock held
> * so cannot fault in missing pages.
> * So fault in the pages first.
> * If they get paged out the inatomic usercopy will fail
> * and the whole operation is retried.
> *
> * Hopefully there are enough memory pages available to
> * stop this looping forever.
> */
What about the other 4 or 5 copies of this loop in the kernel?
This is a pattern, not a one off implementation. Comments describing
how the pattern works belong in the API documentation, not on a
single implemenation of the pattern...
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
prev parent reply other threads:[~2021-06-22 22:20 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-22 15:20 Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? David Howells
2021-06-22 15:27 ` Al Viro
2021-06-22 15:36 ` Al Viro
2021-06-22 17:25 ` Matthew Wilcox
2021-06-22 17:39 ` Linus Torvalds
2021-06-22 17:55 ` David Howells
2021-06-22 18:04 ` Matthew Wilcox
2021-06-22 18:07 ` Linus Torvalds
2021-06-22 18:16 ` Nadav Amit
2021-06-22 18:23 ` Matthew Wilcox
2021-06-22 18:28 ` Linus Torvalds
2021-06-22 18:36 ` Matthew Wilcox
2021-06-22 18:51 ` Nadav Amit
2021-06-22 18:57 ` Linus Torvalds
2021-06-22 18:23 ` David Howells
2021-06-22 18:32 ` Linus Torvalds
2021-06-22 18:13 ` David Howells
2021-06-22 15:32 ` Linus Torvalds
2021-06-22 15:53 ` Linus Torvalds
2021-06-22 15:32 ` Matthew Wilcox
2021-06-22 16:27 ` David Howells
2021-06-22 21:55 ` David Laight
2021-06-22 22:04 ` Matthew Wilcox
2021-06-22 22:31 ` David Laight
2021-06-22 22:20 ` Dave Chinner [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210622222044.GI2419729@dread.disaster.area \
--to=david@fromorbit.com \
--cc=David.Laight@aculab.com \
--cc=akpm@linux-foundation.org \
--cc=dave.hansen@linux.intel.com \
--cc=dhowells@redhat.com \
--cc=linux-ext4@vger.kernel.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=torvalds@linux-foundation.org \
--cc=tytso@mit.edu \
--cc=viro@zeniv.linux.org.uk \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).