[PATCH v5] block/file-posix.c: Use pwritev2() with RWF_DSYNC for FUA

[Pinku Deb Nath]

Full Unit Access (FUA) is an optimization where a disk write with the
flag set will be persisted to disk immediately instead of potentially
remaining in the disk's write cache.

This commit address the todo task
for using pwritev2() with RWF_DSYNC in the thread pool section of
raw_co_prw(), if pwritev2() with RWF_DSYNC is available in the host,
which is always the case for Linux kernel >= 4.7.

The intent for FUA is indicated with the BDRV_REQ_FUA flag.
The old code paths are preserved in case BDRV_REQ_FUA is off
or pwritev2() with RWF_DSYNC is not available.

Support for disk writes with FUA is handled in qemu_pwritev_fua(),
which uses pwritev2() with RWF_DSYNC if available, otherwise falls
back to pwritev2() with no flags followed by flush using
handle_aiocb_flush().

If pwritev2() is not implemented, then disk write in the linear FUA
will fallback to pwrite() + handle_aiocb_flush().

Signed-off-by: Pinku Deb Nath <prantoran@gmail.com>

---

v4:
- Add fallback when qemu_pwritev_fua() returns ENOSYS
- Similar fallback was not added for handle_aiocb_rw_vector()
since there is a preadv_present check in handle_aiocb_rw()

v3:
- Changed signature to add fd, iov, nr_iov
- Return -ENOSYS for non-Linux hosts

v2:
- Moved handle_aiocb_flush() into qemu_pwritev_fua()
- In handle_aiocb_rw_linear(), iovec with iovcnt=1 is created
based on the assumption that there will be only one buffer
---
 block/file-posix.c | 68 ++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 56 insertions(+), 12 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 56d1972d15..59bed7866a 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -229,6 +229,7 @@ typedef struct RawPosixAIOData {
             unsigned long op;
         } zone_mgmt;
     };
+    BdrvRequestFlags flags;
 } RawPosixAIOData;
 
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
@@ -1674,6 +1675,20 @@ qemu_pwritev(int fd, const struct iovec *iov, int 
nr_iov, off_t offset)
     return pwritev(fd, iov, nr_iov, offset);
 }
 
+static ssize_t
+qemu_pwritev_fua(int fd, struct iovec *iov, int nr_iov, off_t offset, const 
RawPosixAIOData *aiocb)
+{
+#ifdef RWF_DSYNC
+    return pwritev2(fd, iov, nr_iov, offset, RWF_DSYNC);
+#else
+    ssize_t len = pwritev2(fd, iov, nr_iov, offset, 0);
+    if (len == 0) {
+        len = handle_aiocb_flush(aiocb);
+    }
+    return len;
+#endif
+}
+
 #else
 
 static bool preadv_present = false;
@@ -1690,6 +1705,11 @@ qemu_pwritev(int fd, const struct iovec *iov, int 
nr_iov, off_t offset)
     return -ENOSYS;
 }
 
+static ssize_t
+qemu_pwritev_fua(int fd, struct iovec *iov, int nr_iov, off_t offset, const 
RawPosixAIOData *aiocb)
+{
+    return -ENOSYS;
+}
 #endif
 
 static ssize_t handle_aiocb_rw_vector(RawPosixAIOData *aiocb)
@@ -1698,10 +1718,16 @@ static ssize_t handle_aiocb_rw_vector(RawPosixAIOData 
*aiocb)
 
     len = RETRY_ON_EINTR(
         (aiocb->aio_type & (QEMU_AIO_WRITE | QEMU_AIO_ZONE_APPEND)) ?
-            qemu_pwritev(aiocb->aio_fildes,
-                           aiocb->io.iov,
-                           aiocb->io.niov,
-                           aiocb->aio_offset) :
+            (aiocb->flags &  BDRV_REQ_FUA) ?
+                qemu_pwritev_fua(aiocb->aio_fildes,
+                                aiocb->io.iov,
+                                aiocb->io.niov,
+                                aiocb->aio_offset,
+                                aiocb) :
+                qemu_pwritev(aiocb->aio_fildes,
+                            aiocb->io.iov,
+                            aiocb->io.niov,
+                            aiocb->aio_offset) :
             qemu_preadv(aiocb->aio_fildes,
                           aiocb->io.iov,
                           aiocb->io.niov,
@@ -1727,10 +1753,31 @@ static ssize_t handle_aiocb_rw_linear(RawPosixAIOData 
*aiocb, char *buf)
 
     while (offset < aiocb->aio_nbytes) {
         if (aiocb->aio_type & (QEMU_AIO_WRITE | QEMU_AIO_ZONE_APPEND)) {
-            len = pwrite(aiocb->aio_fildes,
-                         (const char *)buf + offset,
-                         aiocb->aio_nbytes - offset,
-                         aiocb->aio_offset + offset);
+            if (aiocb->flags & BDRV_REQ_FUA) {
+                struct iovec iov = {
+                    .iov_base = buf + offset,
+                    .iov_len = aiocb->aio_nbytes - offset,
+                };
+                len = qemu_pwritev_fua(aiocb->aio_fildes,
+                                    &iov,
+                                    1,
+                                    aiocb->aio_offset + offset,
+                                    aiocb);
+                if (len == -ENOSYS) {
+                    len = pwrite(aiocb->aio_fildes,
+                                (const char *)buf + offset,
+                                aiocb->aio_nbytes - offset,
+                                aiocb->aio_offset + offset);
+                    if (len == 0) {
+                        len = handle_aiocb_flush(aiocb);
+                    }
+                }
+            } else {
+                len = pwrite(aiocb->aio_fildes,
+                            (const char *)buf + offset,
+                            aiocb->aio_nbytes - offset,
+                            aiocb->aio_offset + offset);
+            }
         } else {
             len = pread(aiocb->aio_fildes,
                         buf + offset,
@@ -2539,14 +2586,11 @@ static int coroutine_fn raw_co_prw(BlockDriverState 
*bs, int64_t *offset_ptr,
             .iov            = qiov->iov,
             .niov           = qiov->niov,
         },
+        .flags          = flags,
     };
 
     assert(qiov->size == bytes);
     ret = raw_thread_pool_submit(handle_aiocb_rw, &acb);
-    if (ret == 0 && (flags & BDRV_REQ_FUA)) {
-        /* TODO Use pwritev2() instead if it's available */
-        ret = raw_co_flush_to_disk(bs);
-    }
     goto out; /* Avoid the compiler err of unused label */
 
 out:
-- 
2.43.0