fuse: writepages: crop secondary requests
If writeback happens while fuse is in FUSE_NOWRITE condition, the request
will be queued but not processed immediately (see fuse_flush_writepages()).
Until FUSE_NOWRITE becomes relaxed, more writebacks can happen. They will
be queued as "secondary" requests to that first ("primary") request.
Existing implementation crops only primary request. This is not correct
because a subsequent extending write(2) may increase i_size and then
secondary requests won't be cropped properly. The result would be stale
data written to the server to a file offset where zeros must be.
Similar problem may happen if secondary requests are attached to an
in-flight request that was already cropped.
The patch solves the issue by cropping all secondary requests in
fuse_writepage_end(). Thanks to Miklos for idea.
Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 077b038..1cb303e 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1436,12 +1436,12 @@
}
/* Called under fc->lock, may release and reacquire it */
-static void fuse_send_writepage(struct fuse_conn *fc, struct fuse_req *req)
+static void fuse_send_writepage(struct fuse_conn *fc, struct fuse_req *req,
+ loff_t size)
__releases(fc->lock)
__acquires(fc->lock)
{
struct fuse_inode *fi = get_fuse_inode(req->inode);
- loff_t size = i_size_read(req->inode);
struct fuse_write_in *inarg = &req->misc.write.in;
__u64 data_size = req->num_pages * PAGE_CACHE_SIZE;
@@ -1482,12 +1482,13 @@
{
struct fuse_conn *fc = get_fuse_conn(inode);
struct fuse_inode *fi = get_fuse_inode(inode);
+ size_t crop = i_size_read(inode);
struct fuse_req *req;
while (fi->writectr >= 0 && !list_empty(&fi->queued_writes)) {
req = list_entry(fi->queued_writes.next, struct fuse_req, list);
list_del_init(&req->list);
- fuse_send_writepage(fc, req);
+ fuse_send_writepage(fc, req, crop);
}
}
@@ -1499,12 +1500,37 @@
mapping_set_error(inode->i_mapping, req->out.h.error);
spin_lock(&fc->lock);
while (req->misc.write.next) {
+ struct fuse_conn *fc = get_fuse_conn(inode);
+ struct fuse_write_in *inarg = &req->misc.write.in;
struct fuse_req *next = req->misc.write.next;
req->misc.write.next = next->misc.write.next;
next->misc.write.next = NULL;
list_add(&next->writepages_entry, &fi->writepages);
- list_add_tail(&next->list, &fi->queued_writes);
- fuse_flush_writepages(inode);
+
+ /*
+ * Skip fuse_flush_writepages() to make it easy to crop requests
+ * based on primary request size.
+ *
+ * 1st case (trivial): there are no concurrent activities using
+ * fuse_set/release_nowrite. Then we're on safe side because
+ * fuse_flush_writepages() would call fuse_send_writepage()
+ * anyway.
+ *
+ * 2nd case: someone called fuse_set_nowrite and it is waiting
+ * now for completion of all in-flight requests. This happens
+ * rarely and no more than once per page, so this should be
+ * okay.
+ *
+ * 3rd case: someone (e.g. fuse_do_setattr()) is in the middle
+ * of fuse_set_nowrite..fuse_release_nowrite section. The fact
+ * that fuse_set_nowrite returned implies that all in-flight
+ * requests were completed along with all of their secondary
+ * requests. Further primary requests are blocked by negative
+ * writectr. Hence there cannot be any in-flight requests and
+ * no invocations of fuse_writepage_end() while we're in
+ * fuse_set_nowrite..fuse_release_nowrite section.
+ */
+ fuse_send_writepage(fc, next, inarg->offset + inarg->size);
}
fi->writectr--;
fuse_writepage_finish(fc, req);