linux-kernel - Re: [PATCH v2 2/3] xen/privcmd: Mark pages as dirty

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Date:   Wed, 8 Jul 2020 07:40:14 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     John Hubbard <jhubbard@...dia.com>,
        Souptick Joarder <jrdr.linux@...il.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        sstabellini@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, Paul Durrant <xadimgnik@...il.com>
Subject: Re: [PATCH v2 2/3] xen/privcmd: Mark pages as dirty

On 07.07.20 21:30, John Hubbard wrote:
> On 2020-07-07 04:43, Jürgen Groß wrote:
>> On 07.07.20 13:30, Souptick Joarder wrote:
>>> On Tue, Jul 7, 2020 at 3:08 PM Jürgen Groß <jgross@...e.com> wrote:
> ...
>>>>> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
>>>>> index 33677ea..f6c1543 100644
>>>>> --- a/drivers/xen/privcmd.c
>>>>> +++ b/drivers/xen/privcmd.c
>>>>> @@ -612,8 +612,11 @@ static void unlock_pages(struct page *pages[], 
>>>>> unsigned int nr_pages)
>>>>>    {
>>>>>        unsigned int i;
>>>>>
>>>>> -     for (i = 0; i < nr_pages; i++)
>>>>> +     for (i = 0; i < nr_pages; i++) {
>>>>> +             if (!PageDirty(pages[i]))
>>>>> +                     set_page_dirty_lock(pages[i]);
>>>>
>>>> With put_page() directly following I think you should be able to use
>>>> set_page_dirty() instead, as there is obviously a reference to the page
>>>> existing.
>>>
>>> Patch [3/3] will convert above codes to use 
>>> unpin_user_pages_dirty_lock()
>>> which internally do the same check. So I thought to keep linux-stable 
>>> and
>>> linux-next code in sync. John had a similar concern [1] and later 
>>> agreed to keep
>>> this check.
>>>
>>> Shall I keep this check ?  No ?
> 
> It doesn't matter *too* much, because patch 3/3 fixes up everything by
> changing it all to unpin_user_pages_dirty_lock(). However, there is 
> something
> to be said for having correct interim patches, too. :)  Details:
> 
>>>
>>> [1] 
>>> https://lore.kernel.org/xen-devel/a750e5e5-fd5d-663b-c5fd-261d7c939ba7@nvidia.com/ 
>>>
>>
>> I wasn't referring to checking PageDirty(), but to the use of
>> set_page_dirty_lock().
>>
>> Looking at the comment just before the implementation of
>> set_page_dirty_lock() suggests that it is fine to use set_page_dirty()
>> instead (so not calling lock_page()).
> 
> 
> no no, that's a misreading of the comment. Unless this xen/privcmd code has
> somehow taken a reference on page->mapping->host (which I do *not* think is
> the case), then it is still racy to call set_page_dirty() here. Instead,
> set_page_dirty_lock() should be used.

Ah, okay. Thanks for the clarification.

So you can add my

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen