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

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Message-ID: <3221175.1624375240@warthog.procyon.org.uk>
Date:   Tue, 22 Jun 2021 16:20:40 +0100
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org
cc:     dhowells@...hat.com, Ted Ts'o <tytso@....edu>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>, willy@...radead.org,
        viro@...iv.linux.org.uk, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"?

Hi Linus,

I've been looking at generic_perform_write() with an eye to adapting a version
for network filesystems in general.  I'm wondering if it's actually safe or
whether it needs 00a3d660cbac05af34cca149cb80fb611e916935 reverting, which is
itself a revert of 998ef75ddb5709bbea0bf1506cd2717348a3c647.

Anyway, I was looking at this bit:

	bytes = min_t(unsigned long, PAGE_SIZE - offset,
					iov_iter_count(i));
	...
	if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
		status = -EFAULT;
		break;
	}

	if (fatal_signal_pending(current)) {
		status = -EINTR;
		break;
	}

	status = a_ops->write_begin(file, mapping, pos, bytes, flags,
					&page, &fsdata);
	if (unlikely(status < 0))
		break;

	if (mapping_writably_mapped(mapping))
		flush_dcache_page(page);

	copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);

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.

Also, I've been thinking about Willy's folio/THP stuff that allows bunches of
pages to be glued together into single objects for efficiency.  This is
problematic with the above code because the faultahead is limited to a maximum
of PAGE_SIZE, but we might be wanting to modify a larger object than that.

David