[RFC][PATCH] cifs: Improve use of filemap_get_folios_tag()

[RFC][PATCH] cifs: Improve use of filemap_get_folios_tag()

[David Howells]

[This additional to the "cifs: Fix cifs_writepages_region()" patch that I
posted]

The inefficiency derived from filemap_get_folios_tag() get a batch of
contiguous folios in Vishal's change to afs that got copied into cifs can
be reduced by skipping over those folios that have been passed by the start
position rather than going through the process of locking, checking and
trying to write them.

A similar change would need to be made in afs, in addition to fixing the bugs
there.

There's also a fix in cifs_write_back_from_locked_folio() where it doesn't
return the amount of data dispatched to the server as ->async_writev() just
returns 0 on success.

Signed-off-by: David Howells <dhowells@redhat.com>
---
diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index ebfcaae8c437..bae1a9709e32 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -2839,6 +2839,7 @@ static ssize_t cifs_write_back_from_locked_folio(struct address_space *mapping,
 	free_xid(xid);
 	if (rc == 0) {
 		wbc->nr_to_write = count;
+		rc = len;
 	} else if (is_retryable_error(rc)) {
 		cifs_pages_write_redirty(inode, start, len);
 	} else {
@@ -2873,6 +2874,13 @@ static int cifs_writepages_region(struct address_space *mapping,
 		for (int i = 0; i < nr; i++) {
 			ssize_t ret;
 			struct folio *folio = fbatch.folios[i];
+			unsigned long long fstart;
+
+			fstart = folio_pos(folio); /* May go backwards with THPs */
+			if (fstart < start &&
+			    folio_size(folio) <= start - fstart)
+				continue;
+			start = fstart;
 
 redo_folio:
 			start = folio_pos(folio); /* May regress with THPs */

Re: [RFC][PATCH] cifs: Improve use of filemap_get_folios_tag()

[Linus Torvalds]

On Fri, Feb 24, 2023 at 7:13 AM David Howells <dhowells@redhat.com> wrote:
>
> The inefficiency derived from filemap_get_folios_tag() get a batch of
> contiguous folios in Vishal's change to afs that got copied into cifs can
> be reduced by skipping over those folios that have been passed by the start
> position rather than going through the process of locking, checking and
> trying to write them.

This patch just makes me go "Ugh".

There's something wrong with this code for it to need these games.

That  just makes me convinced that your other patch that just gets rid
of the batching entirely is the right one.

Of course, I'd be even happier if Willy is right and the code could
use the generic write_cache_pages() and avoid all of these things
entirely. I'm not clear on why cifs and afs are being so different in
the first place, and some of the differences are just odd (like that
skip count).

               Linus

Re: [RFC][PATCH] cifs: Improve use of filemap_get_folios_tag()

[David Howells]

Linus Torvalds <torvalds@linux-foundation.org> wrote:

> Of course, I'd be even happier if Willy is right and the code could
> use the generic write_cache_pages() and avoid all of these things
> entirely. I'm not clear on why cifs and afs are being so different in
> the first place, and some of the differences are just odd (like that
> skip count).

The main reason is that write_cache_pages() doesn't (and can't) check
PG_fscache (btrfs uses PG_private_2 for other purposes).  NFS, 9p and ceph,
for the moment, don't cache files that are open for writing, but I'm intending
to change that at some point.  The intention is to unify the writepages code
for at least 9p, afs, ceph and cifs in netfslib in the future.

David