copy_file_range tries to use the OSD 'copy-from' operation, which simply
performs a full object copy. Unfortunately, the implementation of this
system call assumes that stripe_count is always set to 1 and doesn't take
into account that the data may be striped across an object set. If the
file layout has stripe_count different from 1, then the destination file
data will be corrupted.
For example:
Consider a 8 MiB file with 4 MiB object size, stripe_count of 2 and
stripe_size of 2 MiB; the first half of the file will be filled with 'A's
and the second half will be filled with 'B's:
0 4M 8M Obj1 Obj2
+------+------+ +----+ +----+
file: | AAAA | BBBB | | AA | | AA |
+------+------+ |----| |----|
| BB | | BB |
+----+ +----+
If we copy_file_range this file into a new file (which needs to have the
same file layout!), then it will start by copying the object starting at
file offset 0 (Obj1). And then it will copy the object starting at file
offset 4M -- which is Obj1 again.
Unfortunately, the solution for this is to not allow remote object copies
to be performed when the file layout stripe_count is not 1 and simply
fallback to the default (VFS) copy_file_range implementation.
Signed-off-by: Luis Henriques <lhenriques@xxxxxxxx>
---
Hi Jeff,
I hope my understanding of the whole file striping in CephFS is correct;
I had to go re-read the whole thing to refresh my memory.
Anyway, I guess that this is not really the only solution to this
problem, but it's definitely the simplest one. copy_file_range is
already way more complex that I had ever anticipated. I would rather
keep this simple solution instead of adding more complexity and cover
more corner cases. But yeah, we may want to revisit this in the
future...
[OOT: files layout is probably one of the biggest headaches to sort out
the day we want to implement something like FIEMAP on CephFS ;-) ]
Cheers,
--
Luis
fs/ceph/file.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index d277f71abe0b..3b0e6f9eb6a6 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -1957,9 +1957,12 @@ static ssize_t __ceph_copy_file_range(struct file *src_file, loff_t src_off,
return -EOPNOTSUPP;
if ((src_ci->i_layout.stripe_unit != dst_ci->i_layout.stripe_unit) ||
- (src_ci->i_layout.stripe_count != dst_ci->i_layout.stripe_count) ||
- (src_ci->i_layout.object_size != dst_ci->i_layout.object_size))
+ (src_ci->i_layout.stripe_count != 1) ||
+ (dst_ci->i_layout.stripe_count != 1) ||
+ (src_ci->i_layout.object_size != dst_ci->i_layout.object_size)) {
+ dout("Invalid src/dst files layout\n");
return -EOPNOTSUPP;
+ }
if (len < src_ci->i_layout.object_size)
return -EOPNOTSUPP; /* no remote copy will be done */
