Re: [PATCH] mm/page_isolation: don't putback unisolated page

[David Hildenbrand]

On 06.09.21 14:45, Miaohe Lin wrote:
> On 2021/9/6 20:11, David Hildenbrand wrote:
> > On 06.09.21 14:02, David Hildenbrand wrote:
> > > On 04.09.21 11:18, Miaohe Lin wrote:
> > > > If __isolate_free_page() failed, due to zone watermark check, the page is
> > > > still on the free list. But this page will be put back to free list again
> > > > via __putback_isolated_page() now. This may trigger page->flags checks in
> > > > __free_one_page() if PageReported is set. Or we will corrupt the free list
> > > > because list_add() will be called for pages already on another list.
> > > >
> > > > Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated pageblock")
> > > > Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> > > > ---
> > > >     mm/page_isolation.c | 6 ++----
> > > >     1 file changed, 2 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > > > index 9bb562d5d194..7d70d772525c 100644
> > > > --- a/mm/page_isolation.c
> > > > +++ b/mm/page_isolation.c
> > > > @@ -93,10 +93,8 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
> > > >                 buddy_pfn = __find_buddy_pfn(pfn, order);
> > > >                 buddy = page + (buddy_pfn - pfn);
> > > >     -            if (!is_migrate_isolate_page(buddy)) {
> > > > -                __isolate_free_page(page, order);
> > > > -                isolated_page = true;
> > > > -            }
> > > > +            if (!is_migrate_isolate_page(buddy))
> > > > +                isolated_page = !!__isolate_free_page(page, order);
> > > >             }
> > > >         }
> > > >    
> > >
> > > Thanks!
> > >
> > > Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> >
> > To make the confusion perfect (sorry) :D I tripple-checked:
> >
> > In unset_migratetype_isolate() we check that is_migrate_isolate_page(page) holds, otherwise we return.
> >
> > We call __isolate_free_page() only for such pages.
> >
> > __isolate_free_page() won't perform watermark checks on is_migrate_isolate().
> >
> > Consequently, __isolate_free_page() should never fail when called from unset_migratetype_isolate()
> >
> > If that's correct then we  could instead maybe add a VM_BUG_ON() and a comment why this can't fail.
> >
> >
> > Makes sense or am I missing something?
>
> I think you're right. __isolate_free_page() should never fail when called from unset_migratetype_isolate()
> as explained by you. But it might be too fragile to reply on the failure conditions of __isolate_free_page().
> If that changes, VM_BUG_ON() here might trigger unexpectedly. Or am I just over-worried as failure conditions
> of __isolate_free_page() can hardly change?

Maybe

isolated_page = !!__isolate_free_page(page, order);
/*
 * Isolating a free page in an isolated pageblock is expected to always
 * work as watermarks don't apply here.
 */
VM_BUG_ON(isolated_page);


VM_BUG_ON() allows us to detect any issues when testing. Combined with 
the comment it tells everybody messing with __isolate_free_page() what 
we expect in this function.

In production system, we would handle it gracefully.

--
Thanks,

David / dhildenb