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

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Date:   Tue, 7 Jul 2020 13:43:12 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     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, John Hubbard <jhubbard@...dia.com>,
        Paul Durrant <xadimgnik@...il.com>
Subject: Re: [PATCH v2 2/3] xen/privcmd: Mark pages as dirty

On 07.07.20 13:30, Souptick Joarder wrote:
> On Tue, Jul 7, 2020 at 3:08 PM Jürgen Groß <jgross@...e.com> wrote:
>>
>> On 06.07.20 20:16, Souptick Joarder wrote:
>>> pages need to be marked as dirty before unpinned it in
>>> unlock_pages() which was oversight. This is fixed now.
>>>
>>> Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
>>> Suggested-by: John Hubbard <jhubbard@...dia.com>
>>> Cc: John Hubbard <jhubbard@...dia.com>
>>> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>> Cc: Paul Durrant <xadimgnik@...il.com>
>>> ---
>>>    drivers/xen/privcmd.c | 5 ++++-
>>>    1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> 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 ?
> 
> [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()).

Only the transition from get_user_pages_fast() to pin_user_pages_fast()
requires to use the locked version IMO.


Juergen