linux-ext4 - Re: Do we need to unrevert "fs: do not prefault sys

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Date:   Tue, 22 Jun 2021 17:27:01 +0100
From:   David Howells <dhowells@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     dhowells@...hat.com, torvalds@...ux-foundation.org,
        Ted Ts'o <tytso@....edu>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>, willy@...radead.org,
        linux-mm@...ck.org, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"?

Al Viro <viro@...iv.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.

David