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

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Message-ID: <YNIDdgn0m8d2a0P3@zeniv-ca.linux.org.uk>
Date:   Tue, 22 Jun 2021 15:36:22 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Howells <dhowells@...hat.com>
Cc:     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"?

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.