Re: [PATCH v1] mm/balloon_compaction: avoid duplicate page removal

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 18, 2019 at 02:31:14PM +0800, Wei Wang wrote:
> On 07/18/2019 12:31 PM, Michael S. Tsirkin wrote:
> > On Thu, Jul 18, 2019 at 10:23:30AM +0800, Wei Wang wrote:
> > > Fixes: 418a3ab1e778 (mm/balloon_compaction: List interfaces)
> > > 
> > > A #GP is reported in the guest when requesting balloon inflation via
> > > virtio-balloon. The reason is that the virtio-balloon driver has
> > > removed the page from its internal page list (via balloon_page_pop),
> > > but balloon_page_enqueue_one also calls "list_del"  to do the removal.
> > I would add here "this is necessary when it's used from
> > balloon_page_enqueue_list but not when it's called
> > from balloon_page_enqueue".
> > 
> > > So remove the list_del in balloon_page_enqueue_one, and have the callers
> > > do the page removal from their own page lists.
> > > 
> > > Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> > Patch is good but comments need some work.
> > 
> > > ---
> > >   mm/balloon_compaction.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> > > index 83a7b61..1a5ddc4 100644
> > > --- a/mm/balloon_compaction.c
> > > +++ b/mm/balloon_compaction.c
> > > @@ -11,6 +11,7 @@
> > >   #include <linux/export.h>
> > >   #include <linux/balloon_compaction.h>
> > > +/* Callers ensure that @page has been removed from its original list. */
> > This comment does not make sense. E.g. balloon_page_enqueue
> > does nothing to ensure this. And drivers are not supposed
> > to care how the page lists are managed. Pls drop.
> > 
> > Instead please add the following to balloon_page_enqueue:
> > 
> > 
> > 	Note: drivers must not call balloon_page_list_enqueue on
> 
> Probably, you meant balloon_page_enqueue here.

yes

> The description for balloon_page_enqueue also seems incorrect:
> "allocates a new page and inserts it into the balloon page list."
> This function doesn't do any allocation itself.
> Plan to reword it: inserts a new page into the balloon page list."

And maybe
" Page must have been allocated with balloon_page_alloc.".


Also
 * Driver must call it to properly enqueue a balloon pages before definitively

should be
 * Driver must call it to properly enqueue balloon pages before definitively


> 
> Best,
> Wei