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

[Jürgen Groß <jgross@xxxxxxxx>]

On 07.07.20 13:30, Souptick Joarder wrote:
> On Tue, Jul 7, 2020 at 3:08 PM Jürgen Groß <jgross@xxxxxxxx> 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@xxxxxxxxx>
> > > Suggested-by: John Hubbard <jhubbard@xxxxxxxxxx>
> > > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > > Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > > Cc: Paul Durrant <xadimgnik@xxxxxxxxx>
> > > ---
> > >    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@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()).

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


Juergen