Re: [GIT PULL] fscache: I/O API modernisation and netfs helper library

[David Howells <dhowells@redhat.com>]

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

> Does the code not hold a refcount already?

The attached patch will do that.  Note that it's currently based on top of the
patch that drops the PG_fscache alias, so it refers to PG_private_2.

I've run all three patches through xfstests over afs, both with and without a
cache, and Jeff has tested ceph with them.

David
---
commit 803a09110b41b9f6091a517fc8f5c4b15475048c
Author: David Howells <dhowells@redhat.com>
Date:   Wed Feb 10 11:35:15 2021 +0000

    netfs: Hold a ref on a page when PG_private_2 is set
    
    Take a reference on a page when PG_private_2 is set and drop it once the
    bit is unlocked.
    
    Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: David Howells <dhowells@redhat.com>

diff --git a/fs/netfs/read_helper.c b/fs/netfs/read_helper.c
index 9018224693e9..043d96ca2aad 100644
--- a/fs/netfs/read_helper.c
+++ b/fs/netfs/read_helper.c
@@ -10,6 +10,7 @@
 #include <linux/fs.h>
 #include <linux/mm.h>
 #include <linux/pagemap.h>
+#include <linux/pagevec.h>
 #include <linux/slab.h>
 #include <linux/uio.h>
 #include <linux/sched/mm.h>
@@ -230,10 +231,13 @@ static void netfs_rreq_completed(struct netfs_read_request *rreq)
 static void netfs_rreq_unmark_after_write(struct netfs_read_request *rreq)
 {
 	struct netfs_read_subrequest *subreq;
+	struct pagevec pvec;
 	struct page *page;
 	pgoff_t unlocked = 0;
 	bool have_unlocked = false;
 
+	pagevec_init(&pvec);
+
 	rcu_read_lock();
 
 	list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
@@ -247,6 +251,8 @@ static void netfs_rreq_unmark_after_write(struct netfs_read_request *rreq)
 				continue;
 			unlocked = page->index;
 			unlock_page_private_2(page);
+			if (pagevec_add(&pvec, page) == 0)
+				pagevec_release(&pvec);
 			have_unlocked = true;
 		}
 	}
@@ -403,8 +409,10 @@ static void netfs_rreq_unlock(struct netfs_read_request *rreq)
 				pg_failed = true;
 				break;
 			}
-			if (test_bit(NETFS_SREQ_WRITE_TO_CACHE, &subreq->flags))
+			if (test_bit(NETFS_SREQ_WRITE_TO_CACHE, &subreq->flags)) {
+				get_page(page);
 				SetPagePrivate2(page);
+			}
 			pg_failed |= subreq_failed;
 			if (pgend < iopos + subreq->len)
 				break;