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Re: [PATCH v4 13/16] block/io: support int64_t bytes in bdrv_aligned_pre

From: Eric Blake
Subject: Re: [PATCH v4 13/16] block/io: support int64_t bytes in bdrv_aligned_preadv()
Date: Fri, 22 Jan 2021 10:54:41 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 12/11/20 12:39 PM, Vladimir Sementsov-Ogievskiy wrote:
> 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
> error).
> So, prepare bdrv_aligned_preadv() now.
> Make byte variable in bdrv_padding_rmw_read() int64_t, as it defined
> only to be passed to bdrv_aligned_preadv().

Reads awkwardly, how about:

Make the byte variable in bdrv_padding_rmw_read() int64_t, as it is only
used for pass-through to bdrv_aligned_preadv().

> All bdrv_aligned_preadv() callers are safe as type is widening. Let's
> look inside:
>  - add a new-style assertion that request is good.
>  - callees bdrv_is_allocated(), bdrv_co_do_copy_on_readv() supports
>    int64_t bytes
>  - conversion of bytes_remaining is OK, as we never has requests


>    overflowing BDRV_MAX_LENGTH
>  - looping through bytes_remaining is ok, num is updated to int64_t
>    - for bdrv_driver_preadv we have same limit of max_transfer
>    - qemu_iovec_memset is OK, as bytes+qiov_offset should not overflow
>      qiov->size anyway (thanks to bdrv_check_qiov_request())
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/io.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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