netfs: Replace PG_fscache by setting folio->private and marking dirty

When dirty data is being written to the cache, setting/waiting on/clearing
the fscache flag is always done in tandem with setting/waiting on/clearing
the writeback flag.  The netfslib buffered write routines wait on and set
both flags and the write request cleanup clears both flags, so the fscache
flag is almost superfluous.

The reason it isn't superfluous is because the fscache flag is also used to
indicate that data just read from the server is being written to the cache.
The flag is used to prevent a race involving overlapping direct-I/O writes
to the cache.

Change this to indicate that a page is in need of being copied to the cache
by placing a magic value in folio->private and marking the folios dirty.
Then when the writeback code sees a folio marked in this way, it only
writes it to the cache and not to the server.

If a folio that has this magic value set is modified, the value is just
replaced and the folio will then be uplodaded too.

With this, PG_fscache is no longer required by the netfslib core, 9p and
afs.

Ceph and nfs, however, still need to use the old PG_fscache-based tracking.
To deal with this, a flag, NETFS_ICTX_USE_PGPRIV2, now has to be set on the
flags in the netfs_inode struct for those filesystems.  This reenables the
use of PG_fscache in that inode.  9p and afs use the netfslib write helpers
so get switched over; cifs, for the moment, does page-by-page manual access
to the cache, so doesn't use PG_fscache and is unaffected.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: Matthew Wilcox (Oracle) <willy@infradead.org>
cc: Eric Van Hensbergen <ericvh@kernel.org>
cc: Latchesar Ionkov <lucho@ionkov.net>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Christian Schoenebeck <linux_oss@crudebyte.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Ilya Dryomov <idryomov@gmail.com>
cc: Xiubo Li <xiubli@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
cc: Shyam Prasad N <sprasad@microsoft.com>
cc: Tom Talpey <tom@talpey.com>
cc: Bharath SM <bharathsm@microsoft.com>
cc: Trond Myklebust <trond.myklebust@hammerspace.com>
cc: Anna Schumaker <anna@kernel.org>
cc: netfs@lists.linux.dev
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: ceph-devel@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org

1 files changed, 26 insertions, 10 deletions

diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
index 3298c29b5548..6d49319c82c6 100644
--- a/fs/netfs/buffered_read.c
+++ b/fs/netfs/buffered_read.c
@@ -10,8 +10,11 @@
 #include "internal.h"
 
 /*
- * Unlock the folios in a read operation.  We need to set PG_fscache on any
+ * Unlock the folios in a read operation.  We need to set PG_writeback on any
  * folios we're going to write back before we unlock them.
+ *
+ * Note that if the deprecated NETFS_RREQ_USE_PGPRIV2 is set then we use
+ * PG_private_2 and do a direct write to the cache from here instead.
  */
 void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 {
@@ -48,14 +51,14 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 	xas_for_each(&xas, folio, last_page) {
 		loff_t pg_end;
 		bool pg_failed = false;
-		bool folio_started;
+		bool wback_to_cache = false;
+		bool folio_started = false;
 
 		if (xas_retry(&xas, folio))
 			continue;
 
 		pg_end = folio_pos(folio) + folio_size(folio) - 1;
 
-		folio_started = false;
 		for (;;) {
 			loff_t sreq_end;
 
@@ -63,10 +66,16 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 				pg_failed = true;
 				break;
 			}
-			if (!folio_started && test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags)) {
-				trace_netfs_folio(folio, netfs_folio_trace_copy_to_cache);
-				folio_start_fscache(folio);
-				folio_started = true;
+			if (test_bit(NETFS_RREQ_USE_PGPRIV2, &rreq->flags)) {
+				if (!folio_started && test_bit(NETFS_SREQ_COPY_TO_CACHE,
+							       &subreq->flags)) {
+					trace_netfs_folio(folio, netfs_folio_trace_copy_to_cache);
+					folio_start_fscache(folio);
+					folio_started = true;
+				}
+			} else {
+				wback_to_cache |=
+					test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags);
 			}
 			pg_failed |= subreq_failed;
 			sreq_end = subreq->start + subreq->len - 1;
@@ -98,6 +107,11 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 				kfree(finfo);
 			}
 			folio_mark_uptodate(folio);
+			if (wback_to_cache && !WARN_ON_ONCE(folio_get_private(folio) != NULL)) {
+				trace_netfs_folio(folio, netfs_folio_trace_copy_to_cache);
+				folio_attach_private(folio, NETFS_FOLIO_COPY_TO_CACHE);
+				filemap_dirty_folio(folio->mapping, folio);
+			}
 		}
 
 		if (!test_bit(NETFS_RREQ_DONT_UNLOCK_FOLIOS, &rreq->flags)) {
@@ -491,9 +505,11 @@ retry:
 	netfs_put_request(rreq, false, netfs_rreq_trace_put_return);
 
 have_folio:
-	ret = folio_wait_fscache_killable(folio);
-	if (ret < 0)
-		goto error;
+	if (test_bit(NETFS_ICTX_USE_PGPRIV2, &ctx->flags)) {
+		ret = folio_wait_fscache_killable(folio);
+		if (ret < 0)
+			goto error;
+	}
 have_folio_no_wait:
 	*_folio = folio;
 	_leave(" = 0");