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

From: Stefano Garzarella
Subject: Re: [PATCH v2] block/gluster: correctly set max_pdiscard
Date: Thu, 12 May 2022 18:05:34 +0200

On Thu, May 12, 2022 at 05:44:13PM +0200, Stefano Garzarella wrote:
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 
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?

Okay, I just saw Eric's answer to v1 and I think this is right.

Please explain it in the commit message and also the initial problem that is SIZE_MAX on a 64-bit platform is a negative number for int64_t, so the assert fails.


(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),
   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?


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