[GEDI] [PATCH v3 05/17] block/io: support int64_t bytes in bdrv_co_do_pwrite_zeroes()

Vladimir Sementsov-Ogievskiy vsementsov at virtuozzo.com
Thu Apr 30 11:10:21 UTC 2020

We are generally moving to int64_t for both offset and bytes parameters
on all io paths.

Main motivation is realization of 64-bit write_zeroes operation for
fast zeroing large disk chunks, up to the whole disk.

We chose signed type, to be consistent with off_t (which is signed) and
with possibility for signed return type (where negative value means

So, prepare bdrv_co_do_pwrite_zeroes() now.

Patch-correctness audit by Eric Blake:

  Widens from 32- to 64-bit.  Callers:

    bdrv_co_do_copy_on_readv() - passes 'int64_t pnum' bounded by
      fragmenting loop limited to MAX_BOUNCE_BUFFER
    bdrv_aligned_pwritev() - passes 'unsigned int bytes' - latent bug
      fix for sizes between 2G and 4G, if any

    to see if that bug could be tickled, look at callers of

    bdrv_co_do_zero_pwritev() - splits 'unsigned int bytes' into
      head|body|tail; head and tail are safe but body could be > 2G
    bdrv_co_pwritev_part() - gates with bdrv_check_byte_request()

    continuing the audit, callers of bdrv_co_do_zero_pwritev:

    bdrv_co_pwritev_part() - gates with bdrv_check_byte_request()

    okay, all callers pass < 2G per our current code in
    bdrv_check_byte_request(), so there is no actual bug.

  Use of 'bytes' within the function:

    compute 'int tail' via % 'int alignment' - safe
    fragmentation loop 'int num' - still fragments with a cap on

    use of 'num' within the loop
    compute 'int head' via % 'int alignment' - safe
    clamp size by 'int max_write_zeroes' - safe
    drv->bdrv_co_pwrite_zeroes(int) - safe because of clamping
    clamp size by 'int max_transfer' - safe
    qemu_iovec_init_buf(size_t) - safe because of clamping
    bdrv_driver_pwritev(uint64_t) [well, int64_t after 4/17] - safe

    So even with the wider type, we aren't exceeding the contract of
    anything we pass it on to.  Later patches may improve
    drv->bdrv_co_pwrite_zeroes and qemu_iovec_init_buf to be 64-bit
    clean, at which point we would want to revisit this function to use
    64-bit clamping rather than 32-bit clamping, but it does not have
    to happen here.

Series: 64bit-block-status
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov at virtuozzo.com>
Reviewed-by: Eric Blake <eblake at redhat.com>
 block/io.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/block/io.c b/block/io.c
index eeba3b828c..b83749cc50 100644
--- a/block/io.c
+++ b/block/io.c
@@ -42,7 +42,7 @@
 static void bdrv_parent_cb_resize(BlockDriverState *bs);
 static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
-    int64_t offset, int bytes, BdrvRequestFlags flags);
+    int64_t offset, int64_t bytes, BdrvRequestFlags flags);
 static void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
                                       bool ignore_bds_parents)
@@ -1575,7 +1575,7 @@ int coroutine_fn bdrv_co_preadv_part(BdrvChild *child,
 static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
-    int64_t offset, int bytes, BdrvRequestFlags flags)
+    int64_t offset, int64_t bytes, BdrvRequestFlags flags)
     BlockDriver *drv = bs->drv;
     QEMUIOVector qiov;
@@ -1605,7 +1605,7 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
     assert(max_write_zeroes >= bs->bl.request_alignment);
     while (bytes > 0 && !ret) {
-        int num = bytes;
+        int64_t num = bytes;
         /* Align request.  Block drivers can expect the "bulk" of the request
          * to be aligned, and that unaligned requests do not cross cluster

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