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Tue, 24 Mar 2026 01:43:48 -0700 (PDT) X-Received: by 2002:a05:600c:698e:b0:486:f4d2:eac6 with SMTP id 5b1f17b1804b1-486fedd80c9mr193786915e9.13.1774341827663; Tue, 24 Mar 2026 01:43:47 -0700 (PDT) From: Hanna Czenczek To: qemu-block@nongnu.org Cc: qemu-devel@nongnu.org, Hanna Czenczek , Kevin Wolf , Julia Suvorova , Aarushi Mehta , Stefan Hajnoczi , Stefano Garzarella Subject: [PATCH for-11.0 v2 2/3] linux-aio: Resubmit tails of short reads/writes Date: Tue, 24 Mar 2026 09:43:35 +0100 Message-ID: <20260324084338.37453-3-hreitz@redhat.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260324084338.37453-1-hreitz@redhat.com> References: <20260324084338.37453-1-hreitz@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1774341877320154100 Content-Type: text/plain; charset="utf-8" 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 =3D FUSE_ASYNC_READ | FUSE_ASYNC_DIO; + const uint32_t supported_flags =3D FUSE_ASYNC_READ; if (in->major !=3D 7) { error_report("FUSE major version mismatch: We have 7, but kernel h= as %" @@ -1060,6 +1060,8 @@ fuse_co_read(FuseExport *exp, void **bufptr, uint64_t= offset, uint32_t size) void *buf; int ret; + size =3D 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 =3D 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=3Draw size=3D8192 $ ./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=3Dthreads, short I/O works: $ storage-daemon/qemu-storage-daemon \ --blockdev file,node-name=3Dtest,filename=3Dtest.raw \ --export fuse,id=3Dexp,node-name=3Dtest,mountpoint=3Dtest.raw,writable= =3Dtrue Other shell: $ ./qemu-io --image-opts -c 'read -P 42 0 8k' \ driver=3Dfile,filename=3Dtest.raw,cache.direct=3Don,aio=3Dthreads 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=3Dfile,filename=3Dtest.raw,cache.direct=3Don,aio=3Dthreads 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=3Dnative, it does not: $ ./qemu-io --image-opts -c 'read -P 23 0 8k' \ driver=3Dfile,filename=3Dtest.raw,cache.direct=3Don,aio=3Dnative 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=3Dfile,filename=3Dtest.raw,cache.direct=3Don,aio=3Dnative 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 Signed-off-by: Hanna Czenczek --- block/linux-aio.c | 56 ++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 50 insertions(+), 6 deletions(-) diff --git a/block/linux-aio.c b/block/linux-aio.c index 3843f45eac..0a7424fbb3 100644 --- a/block/linux-aio.c +++ b/block/linux-aio.c @@ -45,6 +45,10 @@ struct qemu_laiocb { size_t nbytes; QEMUIOVector *qiov; =20 + /* For handling short reads/writes */ + size_t total_done; + QEMUIOVector resubmit_qiov; + int fd; int type; BdrvRequestFlags flags; @@ -74,28 +78,61 @@ struct LinuxAioState { }; =20 static void ioq_submit(LinuxAioState *s); +static int laio_do_submit(struct qemu_laiocb *laiocb); =20 static inline ssize_t io_event_ret(struct io_event *ev) { return (ssize_t)(((uint64_t)ev->res2 << 32) | ev->res); } =20 +/** + * Retry tail of short requests. + */ +static int laio_resubmit_short_io(struct qemu_laiocb *laiocb, size_t done) +{ + QEMUIOVector *resubmit_qiov =3D &laiocb->resubmit_qiov; + + laiocb->total_done +=3D 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_d= one); + + return laio_do_submit(laiocb); +} + /* * Completes an AIO request. */ static void qemu_laio_process_completion(struct qemu_laiocb *laiocb) { - int ret; + ssize_t ret; =20 ret =3D laiocb->ret; if (ret !=3D -ECANCELED) { - if (ret =3D=3D laiocb->nbytes) { + if (ret =3D=3D laiocb->nbytes - laiocb->total_done) { ret =3D 0; + } else if (ret > 0 && (laiocb->type =3D=3D QEMU_AIO_READ || + laiocb->type =3D=3D QEMU_AIO_WRITE)) { + ret =3D laio_resubmit_short_io(laiocb, ret); + if (!ret) { + return; + } } else if (ret >=3D 0) { - /* Short reads mean EOF, pad with zeros. */ + /* + * For normal reads and writes, we only get here if ret =3D=3D= 0, which + * means EOF for reads and ENOSPC for writes. + * For zone-append, we get here with any ret >=3D 0, which we = just + * treat as ENOSPC, too (safer than resubmitting, probably, bu= t not + * 100 % clear). + */ if (laiocb->type =3D=3D 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 =3D -ENOSPC; } @@ -103,6 +140,9 @@ static void qemu_laio_process_completion(struct qemu_la= iocb *laiocb) } =20 laiocb->ret =3D ret; + if (laiocb->resubmit_qiov.iov) { + qemu_iovec_destroy(&laiocb->resubmit_qiov); + } =20 /* * 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 =3D &laiocb->iocb; QEMUIOVector *qiov =3D laiocb->qiov; int fd =3D laiocb->fd; - off_t offset =3D laiocb->offset; + off_t offset =3D laiocb->offset + laiocb->total_done; + + if (laiocb->resubmit_qiov.iov) { + qiov =3D &laiocb->resubmit_qiov; + } =20 switch (laiocb->type) { case QEMU_AIO_WRITE: --=20 2.53.0