Re: [Qemu-devel] [PATCH] block: workaround for unaligned byte range in fallocate()

[Vladimir Sementsov-Ogievskiy]

22.08.2019 21:31, Andrey Shinkevich wrote:
> Revert the commit 118f99442d 'block/io.c: fix for the allocation failure'
> and make better error handling for the file systems that do not support
> fallocate() for the unaligned byte range. Allow falling back to pwrite
> in case fallocate() returns EINVAL.
> 
> Suggested-by: Kevin Wolf <address@hidden>
> Suggested-by: Eric Blake <address@hidden>
> Signed-off-by: Andrey Shinkevich <address@hidden>
> ---
> Discussed in email thread with the message ID
> <address@hidden>
> 
>   block/file-posix.c | 7 +++++++
>   block/io.c         | 2 +-
>   2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index fbeb006..2c254ff 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1588,6 +1588,13 @@ static int handle_aiocb_write_zeroes(void *opaque)
>       if (s->has_write_zeroes) {
>           int ret = do_fallocate(s->fd, FALLOC_FL_ZERO_RANGE,
>                                  aiocb->aio_offset, aiocb->aio_nbytes);
> +        if (ret == -EINVAL) {
> +            /*
> +             * Allow falling back to pwrite for file systems that
> +             * do not support fallocate() for unaligned byte range.
> +             */
> +            return -ENOTSUP;
> +        }
>           if (ret == 0 || ret != -ENOTSUP) {
>               return ret;
>           }

Hmm stop, you've done exactly what Den was afraid of:

the next line
   s->has_write_zeroes = false;

will disable write_zeroes forever.

Something like

--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1588,10 +1588,12 @@ static int handle_aiocb_write_zeroes(void *opaque)
      if (s->has_write_zeroes) {
          int ret = do_fallocate(s->fd, FALLOC_FL_ZERO_RANGE,
                                 aiocb->aio_offset, aiocb->aio_nbytes);
-        if (ret == 0 || ret != -ENOTSUP) {
+        if (ret == 0 || (ret != -ENOTSUP && ret != -EINVAL)) {
              return ret;
          }
-        s->has_write_zeroes = false;
+        if (ret == -ENOTSUP) {
+            s->has_write_zeroes = false;
+        }
      }
  #endif


will work better. So, handle ENOTSUP as "disable write_zeros forever", and 
EINVAL as
"don't disable, but fallback to writing zeros". And we need same handling for 
following do_fallocate() calls
too (otherwise they again fails with EINVAL which will break the whole thing).

> diff --git a/block/io.c b/block/io.c
> index 56bbf19..58f08cd 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1558,7 +1558,7 @@ static int coroutine_fn 
> bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
>               assert(!bs->supported_zero_flags);
>           }
>   
> -        if (ret < 0 && !(flags & BDRV_REQ_NO_FALLBACK)) {
> +        if (ret == -ENOTSUP && !(flags & BDRV_REQ_NO_FALLBACK)) {
>               /* Fall back to bounce buffer if write zeroes is unsupported */
>               BdrvRequestFlags write_flags = flags & ~BDRV_REQ_ZERO_WRITE;
>   
> 


-- 
Best regards,
Vladimir