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linux-aio: Resubmit tails of short reads/writes

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Short reads/writes can happen.  One way to reproduce them is via our
FUSE export, with the following diff applied (%s/escaped // to apply --
if you put plain diffs in commit messages, git-am will apply them, and I
would rather avoid breaking FUSE accidentally via this patch):

escaped diff --git a/block/export/fuse.c b/block/export/fuse.c
escaped index a2a478d293..67dc50a412 100644
escaped --- a/block/export/fuse.c
escaped +++ b/block/export/fuse.c
@@ -828,7 +828,7 @@ static ssize_t coroutine_fn GRAPH_RDLOCK
 fuse_co_init(FuseExport *exp, struct fuse_init_out *out,
              const struct fuse_init_in_compat *in)
 {
-    const uint32_t supported_flags = FUSE_ASYNC_READ | FUSE_ASYNC_DIO;
+    const uint32_t supported_flags = FUSE_ASYNC_READ;

     if (in->major != 7) {
         error_report("FUSE major version mismatch: We have 7, but kernel has %"
@@ -1060,6 +1060,8 @@ fuse_co_read(FuseExport *exp, void **bufptr, uint64_t offset, uint32_t size)
     void *buf;
     int ret;

+    size = MIN(size, 4096);
+
     /* Limited by max_read, should not happen */
     if (size > FUSE_MAX_READ_BYTES) {
         return -EINVAL;
@@ -1110,6 +1112,8 @@ fuse_co_write(FuseExport *exp, struct fuse_write_out *out,
     int64_t blk_len;
     int ret;

+    size = MIN(size, 4096);
+
     QEMU_BUILD_BUG_ON(FUSE_MAX_WRITE_BYTES > BDRV_REQUEST_MAX_BYTES);
     /* Limited by max_write, should not happen */
     if (size > FUSE_MAX_WRITE_BYTES) {

Then:
$ ./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

With aio=threads, short I/O works:
$ 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

Other shell:
$ ./qemu-io --image-opts -c 'read -P 42 0 8k' \
    driver=file,filename=test.raw,cache.direct=on,aio=threads
read 8192/8192 bytes at offset 0
8 KiB, 1 ops; 00.00 sec (36.563 MiB/sec and 4680.0923 ops/sec)
$ ./qemu-io --image-opts -c 'write -P 23 0 8k' \
    driver=file,filename=test.raw,cache.direct=on,aio=threads
wrote 8192/8192 bytes at offset 0
8 KiB, 1 ops; 00.00 sec (35.995 MiB/sec and 4607.2970 ops/sec)
$ hexdump -C test.raw
00000000  17 17 17 17 17 17 17 17  17 17 17 17 17 17 17 17  |................|
*
00002000

But with aio=native, it does not:
$ ./qemu-io --image-opts -c 'read -P 23 0 8k' \
    driver=file,filename=test.raw,cache.direct=on,aio=native
Pattern verification failed at offset 0, 8192 bytes
read 8192/8192 bytes at offset 0
8 KiB, 1 ops; 00.00 sec (86.155 MiB/sec and 11027.7900 ops/sec)
$ ./qemu-io --image-opts -c 'write -P 42 0 8k' \
    driver=file,filename=test.raw,cache.direct=on,aio=native
write failed: No space left on device
$ hexdump -C test.raw
00000000  2a 2a 2a 2a 2a 2a 2a 2a  2a 2a 2a 2a 2a 2a 2a 2a  |****************|
*
00001000  17 17 17 17 17 17 17 17  17 17 17 17 17 17 17 17  |................|
*
00002000

This patch fixes that.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20260324084338.37453-3-hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Hanna Czenczek
2026-03-24 09:43:35 +01:00
committed by Kevin Wolf
parent cc03b62df4
commit 7eca3d4883
1 changed files with 50 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files

56
block/linux-aio.c
View File

@@ -45,6 +45,10 @@ struct qemu_laiocb {
size_t nbytes;
QEMUIOVector *qiov;
/* For handling short reads/writes */
size_t total_done;
QEMUIOVector resubmit_qiov;
int fd;
int type;
BdrvRequestFlags flags;
@@ -74,28 +78,61 @@ struct LinuxAioState {
};
static void ioq_submit(LinuxAioState *s);
static int laio_do_submit(struct qemu_laiocb *laiocb);
static inline ssize_t io_event_ret(struct io_event *ev)
{
return (ssize_t)(((uint64_t)ev->res2 << 32) | ev->res);
}
/**
* Retry tail of short requests.
*/
static int laio_resubmit_short_io(struct qemu_laiocb *laiocb, size_t done)
{
QEMUIOVector *resubmit_qiov = &laiocb->resubmit_qiov;
laiocb->total_done += done;
if (!resubmit_qiov->iov) {
qemu_iovec_init(resubmit_qiov, laiocb->qiov->niov);
} else {
qemu_iovec_reset(resubmit_qiov);
}
qemu_iovec_concat(resubmit_qiov, laiocb->qiov,
laiocb->total_done, laiocb->nbytes - laiocb->total_done);
return laio_do_submit(laiocb);
}
/*
* Completes an AIO request.
*/
static void qemu_laio_process_completion(struct qemu_laiocb *laiocb)
{
int ret;
ssize_t ret;
ret = laiocb->ret;
if (ret != -ECANCELED) {
if (ret == laiocb->nbytes) {
if (ret == laiocb->nbytes - laiocb->total_done) {
ret = 0;
} else if (ret > 0 && (laiocb->type == QEMU_AIO_READ ||
laiocb->type == QEMU_AIO_WRITE)) {
ret = laio_resubmit_short_io(laiocb, ret);
if (!ret) {
return;
}
} else if (ret >= 0) {
/* Short reads mean EOF, pad with zeros. */
/*
* For normal reads and writes, we only get here if ret == 0, which
* means EOF for reads and ENOSPC for writes.
* For zone-append, we get here with any ret >= 0, which we just
* treat as ENOSPC, too (safer than resubmitting, probably, but not
* 100 % clear).
*/
if (laiocb->type == QEMU_AIO_READ) {
qemu_iovec_memset(laiocb->qiov, ret, 0,
laiocb->qiov->size - ret);
qemu_iovec_memset(laiocb->qiov, laiocb->total_done, 0,
laiocb->qiov->size - laiocb->total_done);
} else {
ret = -ENOSPC;
}
@@ -103,6 +140,9 @@ static void qemu_laio_process_completion(struct qemu_laiocb *laiocb)
}
laiocb->ret = ret;
if (laiocb->resubmit_qiov.iov) {
qemu_iovec_destroy(&laiocb->resubmit_qiov);
}
/*
* If the coroutine is already entered it must be in ioq_submit() and
@@ -379,7 +419,11 @@ static int laio_do_submit(struct qemu_laiocb *laiocb)
struct iocb *iocbs = &laiocb->iocb;
QEMUIOVector *qiov = laiocb->qiov;
int fd = laiocb->fd;
off_t offset = laiocb->offset;
off_t offset = laiocb->offset + laiocb->total_done;
if (laiocb->resubmit_qiov.iov) {
qiov = &laiocb->resubmit_qiov;
}
switch (laiocb->type) {
case QEMU_AIO_WRITE: