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cf9cdaea6e
Short writes can happen, too, not just short reads. The difference to
aio=native is that the kernel will actually retry the tail of short
requests internally already -- so it is harder to reproduce. But if the
tail of a short request returns an error to the kernel, we will see it
in userspace still. To reproduce this, apply the following patch on top
of the one shown in HEAD^ (again %s/escaped // to apply):
escaped diff --git a/block/export/fuse.c b/block/export/fuse.c
escaped index 67dc50a412..2b98489a32 100644
escaped --- a/block/export/fuse.c
escaped +++ b/block/export/fuse.c
@@ -1059,8 +1059,15 @@ fuse_co_read(FuseExport *exp, void **bufptr, uint64_t offset, uint32_t size)
int64_t blk_len;
void *buf;
int ret;
+ static uint32_t error_size;
- size = MIN(size, 4096);
+ if (error_size == size) {
+ error_size = 0;
+ return -EIO;
+ } else if (size > 4096) {
+ error_size = size - 4096;
+ size = 4096;
+ }
/* Limited by max_read, should not happen */
if (size > FUSE_MAX_READ_BYTES) {
@@ -1111,8 +1118,15 @@ fuse_co_write(FuseExport *exp, struct fuse_write_out *out,
{
int64_t blk_len;
int ret;
+ static uint32_t error_size;
- size = MIN(size, 4096);
+ if (error_size == size) {
+ error_size = 0;
+ return -EIO;
+ } else if (size > 4096) {
+ error_size = size - 4096;
+ size = 4096;
+ }
QEMU_BUILD_BUG_ON(FUSE_MAX_WRITE_BYTES > BDRV_REQUEST_MAX_BYTES);
/* Limited by max_write, should not happen */
I know this is a bit artificial because to produce this, there must be
an I/O error somewhere anyway, but if it does happen, qemu will
understand it to mean ENOSPC for short writes, which is incorrect. So I
believe we need to resubmit the tail to maybe have it succeed now, or at
least get the correct error code.
Reproducer as before:
$ ./qemu-img create -f raw test.raw 8k
Formatting 'test.raw', fmt=raw size=8192
$ ./qemu-io -f raw -c 'write -P 42 0 8k' test.raw
wrote 8192/8192 bytes at offset 0
8 KiB, 1 ops; 00.00 sec (64.804 MiB/sec and 8294.9003 ops/sec)
$ hexdump -C test.raw
00000000 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a |****************|
*
00002000
$ storage-daemon/qemu-storage-daemon \
--blockdev file,node-name=test,filename=test.raw \
--export fuse,id=exp,node-name=test,mountpoint=test.raw,writable=true
$ ./qemu-io --image-opts -c 'read -P 23 0 8k' \
driver=file,filename=test.raw,cache.direct=on,aio=io_uring
read 8192/8192 bytes at offset 0
8 KiB, 1 ops; 00.00 sec (58.481 MiB/sec and 7485.5342 ops/sec)
$ ./qemu-io --image-opts -c 'write -P 23 0 8k' \
driver=file,filename=test.raw,cache.direct=on,aio=io_uring
write failed: No space left on device
$ hexdump -C test.raw
00000000 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 |................|
*
00001000 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a |****************|
*
00002000
So short reads already work (because there is code for that), but short
writes incorrectly produce ENOSPC. This patch fixes that by
resubmitting not only the tail of short reads but short writes also.
(And this patch uses the opportunity to make it so qemu_iovec_destroy()
is called only if req->resubmit_qiov.iov is non-NULL. Functionally a
non-op, but this is how the code generally checks whether the
resubmit_qiov has been set up or not.)
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20260324084338.37453-4-hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>