[PATCH 09/12] file-posix: base the zone append limit on the transfer limit

[Niklas Cassel <cassel@kernel.org>]

The zone_append_max_bytes queue attribute is the largest
REQ_OP_ZONE_APPEND that the device accepts, and Linux has no interface
for issuing one from userspace: include/uapi has no such operation, and
every submitter of REQ_OP_ZONE_APPEND is in the kernel. Userspace writes
to a sequential zone with an ordinary write at the write pointer.

That is what this driver does. raw_co_zone_append() substitutes the write
pointer of the zone for the offset and hands the request to raw_co_prw(),
which reaches handle_aiocb_rw_vector() and issues a plain pwritev(). The
kernel never sees a zone append, so the attribute describes a limit on an
operation that is never issued.

Nor is it a limit that this driver runs into. The kernel splits a write
that exceeds the transfer limit rather than refusing it, so a larger
append succeeds: on a null_blk device whose zone_append_max_bytes is
130560, a 16 MiB append completes and advances the write pointer by
16 MiB. Reporting the attribute only understates what the driver can do,
because Linux derives it as a minimum that already includes max_sectors
and chunk_sectors.

Report max_hw_transfer instead, the limit that governs the write the
driver actually issues. There is no use in telling a guest that it may
append more than the device carries in one command, and a frontend then
does not have to reason about how this driver implements an append in
order to bound the value it advertises.

On a null_blk device with max_hw_sectors_kb of 127 and
zone_append_max_bytes of 130560, virtio-blk reports 255 sectors before
and after.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
---
 block/file-posix.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index f267513a4e..e019cc3cd8 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1488,10 +1488,15 @@ static void raw_refresh_zoned_limits(BlockDriverState *bs, struct stat *st,
     }
     bs->bl.nr_zones = ret;
 
-    ret = get_sysfs_long_val(st, "zone_append_max_bytes");
-    if (ret > 0) {
-        bs->bl.max_append_sectors = ret >> BDRV_SECTOR_BITS;
-    }
+    /*
+     * raw_co_zone_append() carries out an append as an ordinary write at the
+     * write pointer, so the zone_append_max_bytes attribute, which bounds an
+     * operation that this driver never issues, does not apply. The kernel
+     * splits a write that is larger than the transfer limit rather than
+     * refusing it, but there is no use in telling a guest that it may append
+     * more than the device carries in one command.
+     */
+    bs->bl.max_append_sectors = bs->bl.max_hw_transfer >> BDRV_SECTOR_BITS;
 
     ret = get_sysfs_long_val(st, "zone_write_granularity");
     if (ret >= 0) {
-- 
2.55.0