[PATCH v2 6/6] ceph: Fix write storm on fscrypted files

[Sam Edwards <cfsworks@gmail.com>]

CephFS stores file data across multiple RADOS objects. An object is the
atomic unit of storage, so the writeback code must clean only folios
that belong to the same object with each OSD request.

CephFS also supports RAID0-style striping of file contents: if enabled,
each object stores multiple unbroken "stripe units" covering different
portions of the file; if disabled, a "stripe unit" is simply the whole
object. The stripe unit is (usually) reported as the inode's block size.

Though the writeback logic could, in principle, lock all dirty folios
belonging to the same object, its current design is to lock only a
single stripe unit at a time. Ever since this code was first written,
it has determined this size by checking the inode's block size.
However, the relatively-new fscrypt support needed to reduce the block
size for encrypted inodes to the crypto block size (see 'fixes' commit),
which causes an unnecessarily high number of write operations (~1024x as
many, with 4MiB objects) and correspondingly degraded performance.

Fix this (and clarify intent) by using i_layout.stripe_unit directly in
ceph_define_write_size() so that encrypted inodes are written back with
the same number of operations as if they were unencrypted.

Fixes: 94af0470924c ("ceph: add some fscrypt guardrails")
Cc: stable@vger.kernel.org
Signed-off-by: Sam Edwards <CFSworks@gmail.com>
---
 fs/ceph/addr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index f2db05b51a3b..b97a6120d4b9 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -1000,7 +1000,8 @@ unsigned int ceph_define_write_size(struct address_space *mapping)
 {
 	struct inode *inode = mapping->host;
 	struct ceph_fs_client *fsc = ceph_inode_to_fs_client(inode);
-	unsigned int wsize = i_blocksize(inode);
+	struct ceph_inode_info *ci = ceph_inode(inode);
+	unsigned int wsize = ci->i_layout.stripe_unit;
 
 	if (fsc->mount_options->wsize < wsize)
 		wsize = fsc->mount_options->wsize;
-- 
2.51.2