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

[Mike Kravetz]

On 09/15/23 10:16, 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.

Thanks!  That makes sense.

Glad my testing (for something else) triggered it.
-- 
Mike Kravetz