cifs: on CONFIG_HIGHMEM machines, limit the rsize/wsize to the kmap space

commit 3ae629d98bd5ed77585a878566f04f310adbc591 upstream.

We currently rely on being able to kmap all of the pages in an async
read or write request. If you're on a machine that has CONFIG_HIGHMEM
set then that kmap space is limited, sometimes to as low as 512 slots.

With 512 slots, we can only support up to a 2M r/wsize, and that's
assuming that we can get our greedy little hands on all of them. There
are other users however, so it's possible we'll end up stuck with a
size that large.

Since we can't handle a rsize or wsize larger than that currently, cap
those options at the number of kmap slots we have. We could consider
capping it even lower, but we currently default to a max of 1M. Might as
well allow those luddites on 32 bit arches enough rope to hang
themselves.

A more robust fix would be to teach the send and receive routines how
to contend with an array of pages so we don't need to marshal up a kvec
array at all. That's a fairly significant overhaul though, so we'll need
this limit in place until that's ready.

Reported-by: Jian Li <jiali@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

1 files changed, 18 insertions, 0 deletions

diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index b21670cd6c8c..56c152d33b22 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -2925,6 +2925,18 @@ void cifs_setup_cifs_sb(struct smb_vol *pvolume_info,
 #define CIFS_DEFAULT_NON_POSIX_RSIZE (60 * 1024)
 #define CIFS_DEFAULT_NON_POSIX_WSIZE (65536)
 
+/*
+ * On hosts with high memory, we can't currently support wsize/rsize that are
+ * larger than we can kmap at once. Cap the rsize/wsize at
+ * LAST_PKMAP * PAGE_SIZE. We'll never be able to fill a read or write request
+ * larger than that anyway.
+ */
+#ifdef CONFIG_HIGHMEM
+#define CIFS_KMAP_SIZE_LIMIT	(LAST_PKMAP * PAGE_CACHE_SIZE)
+#else /* CONFIG_HIGHMEM */
+#define CIFS_KMAP_SIZE_LIMIT	(1<<24)
+#endif /* CONFIG_HIGHMEM */
+
 static unsigned int
 cifs_negotiate_wsize(struct cifs_tcon *tcon, struct smb_vol *pvolume_info)
 {
@@ -2955,6 +2967,9 @@ cifs_negotiate_wsize(struct cifs_tcon *tcon, struct smb_vol *pvolume_info)
 		wsize = min_t(unsigned int, wsize,
 				server->maxBuf - sizeof(WRITE_REQ) + 4);
 
+	/* limit to the amount that we can kmap at once */
+	wsize = min_t(unsigned int, wsize, CIFS_KMAP_SIZE_LIMIT);
+
 	/* hard limit of CIFS_MAX_WSIZE */
 	wsize = min_t(unsigned int, wsize, CIFS_MAX_WSIZE);
 
@@ -2996,6 +3011,9 @@ cifs_negotiate_rsize(struct cifs_tcon *tcon, struct smb_vol *pvolume_info)
 	if (!(server->capabilities & CAP_LARGE_READ_X))
 		rsize = min_t(unsigned int, CIFSMaxBufSize, rsize);
 
+	/* limit to the amount that we can kmap at once */
+	rsize = min_t(unsigned int, rsize, CIFS_KMAP_SIZE_LIMIT);
+
 	/* hard limit of CIFS_MAX_RSIZE */
 	rsize = min_t(unsigned int, rsize, CIFS_MAX_RSIZE);