linux-kernel - Re: [PATCH master] Ignore failure to unmap INVALID_GRANT

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Date:   Sun, 10 Jul 2022 19:19:50 +0000
From:   Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.com>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>
CC:     Linux kernel regressions <regressions@...ts.linux.dev>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        Xen developer discussion <xen-devel@...ts.xenproject.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH master] Ignore failure to unmap INVALID_GRANT_HANDLE


On 08.07.22 19:37, Demi Marie Obenour wrote:

Hello Demi Marie


Please add subsystem to the commit subject, for example "xen/gntdev:".


> 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()")
> ---
>   drivers/xen/gntdev.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 4b56c39f766d4da68570d08d963f6ef40c8d9c37..22fcd503f4a4487d0aed147c94f432683dad8c73 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -396,13 +396,17 @@ 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 &&
> +			map->unmap_ops[offset+i].handle !=
> +			INVALID_GRANT_HANDLE);


Nit: While extending the check I would clarify the first half of it: 
"map->unmap_ops[offset+i].status != GNTST_okay"


>   		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 &&
> +				map->kunmap_ops[offset+i].handle !=
> +				INVALID_GRANT_HANDLE);

ditto



With updated subject:

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


>   			pr_debug("kunmap handle=%u st=%d\n",
>   				 map->kunmap_ops[offset+i].handle,
>   				 map->kunmap_ops[offset+i].status);

-- 
Regards,

Oleksandr Tyshchenko