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

[Johannes Weiner]

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.

---