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
