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Date:   Tue, 22 Jun 2021 15:36:22 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     David Howells <dhowells@redhat.com>
Cc:     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,
        linux-mm@kvack.org, linux-ext4@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user
 buffer pages"?
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References: <3221175.1624375240@warthog.procyon.org.uk>
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Sender: Al Viro <viro@ftp.linux.org.uk>
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On Tue, Jun 22, 2021 at 03:27:43PM +0000, Al Viro 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.

Note that the revert you propose is going to do fault-in anyway; we really can't
avoid it.  The only thing it does is optimistically trying without that the
first time around, which is going to be an overall loss exactly in "slow
write_begin" case.  If source pages are absent, you'll get copyin fail;
iov_iter_copy_from_user_atomic() (or its replacement) is disabling pagefaults
itself.