[PATCH v3] loop: fix no-unmap write-zeroes request behavior

[Darrick J. Wong]

From: Darrick J. Wong <darrick.w...@oracle.com>

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.w...@oracle.com>
---
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);
+               /*
+                * If the caller doesn't want deallocation, call zeroout to
+                * write zeroes the range.  Otherwise, punch them out.
+                */
+               return lo_fallocate(lo, rq, pos,
+                       (rq->cmd_flags & REQ_NOUNMAP) ?
+                               FALLOC_FL_ZERO_RANGE :
+                               FALLOC_FL_PUNCH_HOLE);
+       case REQ_OP_DISCARD:
+               return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);
        case REQ_OP_WRITE:
                if (lo->transfer)
                        return lo_write_transfer(lo, rq, pos);