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

[JÃrgen GroÃ]

On 07.07.20 21:30, John Hubbard wrote:
> On 2020-07-07 04:43, JÃrgen Groà wrote:
> > On 07.07.20 13:30, Souptick Joarder wrote:
> > > On Tue, Jul 7, 2020 at 3:08 PM JÃrgen Groà <jgross@xxxxxxxx> 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)
> > > > > ÂÂ {
> > > > > ÂÂÂÂÂÂ unsigned int i;
> > > > >
> > > > > -ÂÂÂÂ for (i = 0; i < nr_pages; i++)
> > > > > +ÂÂÂÂ for (i = 0; i < nr_pages; i++) {
> > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂ if (!PageDirty(pages[i]))
> > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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. :)Â Details:
>
> > > [1] 
> > > https://lore.kernel.org/xen-devel/a750e5e5-fd5d-663b-c5fd-261d7c939ba7@xxxxxxxxxx/ 
> >
> > 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@xxxxxxxx>


Juergen