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

[David Hildenbrand]

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?

--
Thanks,

David / dhildenb