Re: [PATCH V2 0/6] mm: page_alloc: freelist migratetype hygiene

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Sep 18, 2023 at 09:07:53AM +0200, Vlastimil Babka wrote:
> On 9/15/23 16:16, Johannes Weiner wrote:
> > On Thu, Sep 14, 2023 at 04:52:38PM -0700, Mike Kravetz wrote:
> >> In next-20230913, I started hitting the following BUG.  Seems related
> >> to this series.  And, if series is reverted I do not see the BUG.
> >> 
> >> I can easily reproduce on a small 16G VM.  kernel command line contains
> >> "hugetlb_free_vmemmap=on hugetlb_cma=4G".  Then run the script,
> >> while true; do
> >>  echo 4 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> >>  echo 4 > /sys/kernel/mm/hugepages/hugepages-1048576kB/demote
> >>  echo 0 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
> >> done
> >> 
> >> For the BUG below I believe it was the first (or second) 1G page creation from
> >> CMA that triggered:  cma_alloc of 1G.
> >> 
> >> Sorry, have not looked deeper into the issue.
> > 
> > Thanks for the report, and sorry about the breakage!
> > 
> > I was scratching my head at this:
> > 
> >                         /* MIGRATE_ISOLATE page should not go to pcplists */
> >                         VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
> > 
> > because there is nothing in page isolation that prevents setting
> > MIGRATE_ISOLATE on something that's on the pcplist already. So why
> > didn't this trigger before already?
> > 
> > Then it clicked: it used to only check the *pcpmigratetype* determined
> > by free_unref_page(), which of course mustn't be MIGRATE_ISOLATE.
> > 
> > Pages that get isolated while *already* on the pcplist are fine, and
> > are handled properly:
> > 
> >                         mt = get_pcppage_migratetype(page);
> > 
> >                         /* MIGRATE_ISOLATE page should not go to pcplists */
> >                         VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
> > 
> >                         /* Pageblock could have been isolated meanwhile */
> >                         if (unlikely(isolated_pageblocks))
> >                                 mt = get_pageblock_migratetype(page);
> > 
> > So this was purely a sanity check against the pcpmigratetype cache
> > operations. With that gone, we can remove it.
> 
> Agreed, I assume you'll fold it in 1/6 in v3.

Yes, will do.