linux-kernel - Re: [PATCH v3] xen/gntdev: Ignore failure to unmap INVALID_GRANT

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Message-ID: <b4f411d4-f3b9-4e44-a80a-86b067324a97@epam.com>
Date:   Mon, 11 Jul 2022 06:54:05 +0000
From:   Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.com>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>
CC:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH v3] xen/gntdev: Ignore failure to unmap
 INVALID_GRANT_HANDLE


On 11.07.22 02:05, Demi Marie Obenour wrote:

Hello Demi Marie

> The error paths of gntdev_mmap() can call unmap_grant_pages() even
> though not all of the pages have been successfully mapped.  This will
> trigger the WARN_ON()s in __unmap_grant_pages_done().  The number of
> warnings can be very large; I have observed thousands of lines of
> warnings in the systemd journal.
>
> Avoid this problem by only warning on unmapping failure if the handle
> being unmapped is not INVALID_GRANT_HANDLE.  The handle field of any
> page that was not successfully mapped will be INVALID_GRANT_HANDLE, so
> this catches all cases where unmapping can legitimately fail.
>
> Suggested-by: Juergen Gross <jgross@...e.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.com>
> Fixes: dbe97cff7dd9 ("xen/gntdev: Avoid blocking in unmap_grant_pages()")

Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>


> ---
>   drivers/xen/gntdev.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> Changes since v2:
>
> - Use unmap_ops instead of kunmap_ops in the first WARN_ON
>
> Changes since v1:
>
> - Explicitly check for a status other than GNTST_okay instead of
>    implicitly checking that it is nonzero.
> - Avoid wrapping a line as within a comparison, as this makes the code
>    hard to read.
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 4b56c39f766d4da68570d08d963f6ef40c8d9c37..84b143eef395b1585f3a8c0fdcb301ce9fbc52ec 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -396,13 +396,15 @@ static void __unmap_grant_pages_done(int result,
>   	unsigned int offset = data->unmap_ops - map->unmap_ops;
>   
>   	for (i = 0; i < data->count; i++) {
> -		WARN_ON(map->unmap_ops[offset+i].status);
> +		WARN_ON(map->unmap_ops[offset + i].status != GNTST_okay &&
> +			map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
>   		pr_debug("unmap handle=%d st=%d\n",
>   			map->unmap_ops[offset+i].handle,
>   			map->unmap_ops[offset+i].status);
>   		map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
>   		if (use_ptemod) {
> -			WARN_ON(map->kunmap_ops[offset+i].status);
> +			WARN_ON(map->kunmap_ops[offset + i].status != GNTST_okay &&
> +				map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
>   			pr_debug("kunmap handle=%u st=%d\n",
>   				 map->kunmap_ops[offset+i].handle,
>   				 map->kunmap_ops[offset+i].status);

-- 
Regards,

Oleksandr Tyshchenko