Re: [PATCH v2] block/gluster: correctly set max_pdiscard

[Stefano Garzarella]

On Thu, May 12, 2022 at 12:30:48PM +0200, Fabian Ebner wrote:
> On 64-bit platforms, SIZE_MAX is too large for max_pdiscard, which is

The main problem is that SIZE_MAX for an int64_t is a negative value.

> int64_t, and the following assertion would be triggered:
> qemu-system-x86_64: ../block/io.c:3166: bdrv_co_pdiscard: Assertion
> `max_pdiscard >= bs->bl.request_alignment' failed.
>
> Fixes: 0c8022876f ("block: use int64_t instead of int in driver discard 
> handlers")
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
> ---
> block/gluster.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/block/gluster.c b/block/gluster.c
> index 398976bc66..f711bf0bd6 100644
> --- a/block/gluster.c
> +++ b/block/gluster.c
> @@ -891,7 +891,7 @@ out:
> static void qemu_gluster_refresh_limits(BlockDriverState *bs, Error **errp)
> {
>     bs->bl.max_transfer = GLUSTER_MAX_TRANSFER;
> -    bs->bl.max_pdiscard = SIZE_MAX;
> +    bs->bl.max_pdiscard = MIN(SIZE_MAX, INT64_MAX);

What would be the problem if we use INT64_MAX?
(I guess the intention of the original patch was to set the maximum 
value in drivers that do not have a specific maximum).

Or we can set to 0, since in block/io.c we have this code:

    max_pdiscard = QEMU_ALIGN_DOWN(MIN_NON_ZERO(bs->bl.max_pdiscard, INT64_MAX),
                                   align);
    assert(max_pdiscard >= bs->bl.request_alignment);

Where `max_pdiscard` is set to INT64_MAX (and aligned) if 
bs->bl.max_pdiscard is 0.

> }
>
> static int qemu_gluster_reopen_prepare(BDRVReopenState *state,
> @@ -1304,7 +1304,7 @@ static coroutine_fn int 
> qemu_gluster_co_pdiscard(BlockDriverState *bs,
>     GlusterAIOCB acb;
>     BDRVGlusterState *s = bs->opaque;
>
> -    assert(bytes <= SIZE_MAX); /* rely on max_pdiscard */
> +    assert(bytes <= MIN(SIZE_MAX, INT64_MAX)); /* rely on max_pdiscard */

Can we use bs->bl.max_pdiscard directly here?

Thanks,
Stefano