[Qemu-devel] [PATCH v2 1/1] block: prepare bdrv_co_do_write_zeroes to deal with large bl.max_write_zeroes

[Denis V. Lunev]

bdrv_co_do_write_zeroes split writes using bl.max_write_zeroes or
16 MiB as a chunk size. This is implemented in this way to tolerate
buggy block backends which do not accept too big requests.

Though if the bdrv_co_write_zeroes callback is not good enough, we
fallback to write data explicitely using bdrv_co_writev and we
create buffer to accomodate zeroes inside. The size of this buffer
is the size of the chunk. Thus if the underlying layer will have
bl.max_write_zeroes high enough, f.e. 4 GiB, the allocation can fail.

Actually, there is no need to allocate such a big amount of memory.
We could simply allocate 1 MiB buffer and create iovec, which will
point to the same memory.

Signed-off-by: Denis V. Lunev <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Stefan Hajnoczi <address@hidden>
CC: Peter Lieven <address@hidden>
---
Changes from v1:
- using MAX_WRITE_ZEROES_DEFAULT as a limit for real write

 block.c | 40 +++++++++++++++++++++++++++++-----------
 1 file changed, 29 insertions(+), 11 deletions(-)

diff --git a/block.c b/block.c
index 4165d42..49fd4e3 100644
--- a/block.c
+++ b/block.c
@@ -3173,14 +3173,18 @@ int coroutine_fn bdrv_co_copy_on_readv(BlockDriverState 
*bs,
  * of 32768 512-byte sectors (16 MiB) per request.
  */
 #define MAX_WRITE_ZEROES_DEFAULT 32768
+/* allocate iovec with zeroes using 1 MiB chunks to avoid to big allocations */
+#define MAX_ZEROES_CHUNK (1024 * 1024)
 
 static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
     int64_t sector_num, int nb_sectors, BdrvRequestFlags flags)
 {
     BlockDriver *drv = bs->drv;
     QEMUIOVector qiov;
-    struct iovec iov = {0};
     int ret = 0;
+    void *chunk = NULL;
+
+    qemu_iovec_init(&qiov, 0);
 
     int max_write_zeroes = bs->bl.max_write_zeroes ?
                            bs->bl.max_write_zeroes : MAX_WRITE_ZEROES_DEFAULT;
@@ -3217,27 +3221,40 @@ static int coroutine_fn 
bdrv_co_do_write_zeroes(BlockDriverState *bs,
         }
 
         if (ret == -ENOTSUP) {
+            int64_t num_bytes;
+            int chunk_size;
+
             /* Fall back to bounce buffer if write zeroes is unsupported */
-            iov.iov_len = num * BDRV_SECTOR_SIZE;
-            if (iov.iov_base == NULL) {
-                iov.iov_base = qemu_try_blockalign(bs, num * BDRV_SECTOR_SIZE);
-                if (iov.iov_base == NULL) {
+            num = MIN(num, MAX_WRITE_ZEROES_DEFAULT >> BDRV_SECTOR_BITS);
+
+            num_bytes = (int64_t)num << BDRV_SECTOR_BITS;
+            chunk_size = MIN(MAX_ZEROES_CHUNK, num_bytes);
+
+            if (chunk == NULL) {
+                chunk = qemu_try_blockalign(bs, chunk_size);
+                if (chunk == NULL) {
                     ret = -ENOMEM;
                     goto fail;
                 }
-                memset(iov.iov_base, 0, num * BDRV_SECTOR_SIZE);
+                memset(chunk, 0, chunk_size);
+            }
+
+            while (num_bytes > 0) {
+                int to_add = MIN(chunk_size, num_bytes);
+                qemu_iovec_add(&qiov, chunk, to_add);
+                num_bytes -= to_add;
             }
-            qemu_iovec_init_external(&qiov, &iov, 1);
 
             ret = drv->bdrv_co_writev(bs, sector_num, num, &qiov);
 
             /* Keep bounce buffer around if it is big enough for all
              * all future requests.
              */
-            if (num < max_write_zeroes) {
-                qemu_vfree(iov.iov_base);
-                iov.iov_base = NULL;
+            if (chunk_size != MAX_ZEROES_CHUNK) {
+                qemu_vfree(chunk);
+                chunk = NULL;
             }
+            qemu_iovec_reset(&qiov);
         }
 
         sector_num += num;
@@ -3245,7 +3262,8 @@ static int coroutine_fn 
bdrv_co_do_write_zeroes(BlockDriverState *bs,
     }
 
 fail:
-    qemu_vfree(iov.iov_base);
+    qemu_iovec_destroy(&qiov);
+    qemu_vfree(chunk);
     return ret;
 }
 
-- 
1.9.1