On 10/14/19 10:50 AM, Darrick J. Wong wrote:
From: Darrick J. Wong <darrick.wong@xxxxxxxxxx> Currently, if the loop device receives a WRITE_ZEROES request, it asks the underlying filesystem to punch out the range. This behavior is correct if unmapping is allowed. However, a NOUNMAP request means that the caller doesn't want us to free the storage backing the range, so punching out the range is incorrect behavior. To satisfy a NOUNMAP | WRITE_ZEROES request, loop should ask the underlying filesystem to FALLOC_FL_ZERO_RANGE, which is (according to the fallocate documentation) required to ensure that the entire range is backed by real storage, which suffices for our purposes. Fixes: 19372e2769179dd ("loop: implement REQ_OP_WRITE_ZEROES") Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> --- v3: refactor into a single fallocate function v2: reorganize a little according to hch feedback --- drivers/block/loop.c | 26 ++++++++++++++++++-------- 1 file changed, 18 insertions(+), 8 deletions(-) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index f6f77eaa7217..ef6e251857c8 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -417,18 +417,20 @@ static int lo_read_transfer(struct loop_device *lo, struct request *rq, return ret; }-static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)+static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos, + int mode) { /* - * We use punch hole to reclaim the free space used by the - * image a.k.a. discard. However we do not support discard if - * encryption is enabled, because it may give an attacker - * useful information. + * We use fallocate to manipulate the space mappings used by the image + * a.k.a. discard/zerorange. However we do not support this if + * encryption is enabled, because it may give an attacker useful + * information. */ struct file *file = lo->lo_backing_file; - int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE; int ret;+ mode |= FALLOC_FL_KEEP_SIZE;+ if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) { ret = -EOPNOTSUPP; goto out; @@ -596,9 +598,17 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq) switch (req_op(rq)) { case REQ_OP_FLUSH: return lo_req_flush(lo, rq); - case REQ_OP_DISCARD: case REQ_OP_WRITE_ZEROES: - return lo_discard(lo, rq, pos);
cxz ÿbvVBV
+ case REQ_OP_DISCARD: + return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);
I get lost in the twisty passages. What happens if the filesystem hosting the backing file doesn't support fallocate, and REQ_OP_DISCARD / REQ_OP_WRITE_ZEROES returns EOPNOTSUPP - discard is advisory, is it ok to fail REQ_OP_WRITE_ZEROES? Does something at another layer fall back to writing zeros? -Eric
case REQ_OP_WRITE: if (lo->transfer) return lo_write_transfer(lo, rq, pos);
