mm: Question about the race condition of pages in buddy during isolating pages

["Xu, Yanfei" <yanfei.xu@windriver.com>]

Hi Vlastimil,

When I inspect the the codes about isolating pages, there are some lines
from you make me confused. As blow:

Locate in isolate_migratepages_block()
mm/compaction.c

  908                 /*
  909                  * Skip if free. We read page order here without 
zone lock
  910                  * which is generally unsafe, but the race window 
is small and
  911                  * the worst thing that can happen is that we skip 
some
  912                  * potential isolation targets.
  913                  */
  914                 if (PageBuddy(page)) {
  915                         unsigned long freepage_order = 
buddy_order_unsafe(page);
  916
  917                         /*
  918                          * Without lock, we cannot be sure that 
what we got is
  919                          * a valid page order. Consider only 
values in the
  920                          * valid order range to prevent low_pfn 
overflow.
  921                          */
  922                         if (freepage_order > 0 && freepage_order < 
MAX_ORDER))
  923                                 low_pfn += (1UL << freepage_order) 
- 1;
  924                         continue;
  925                 }

These lines was isntroduced by the commit 99c0fd5e51c("mm, compaction:
skip buddy pages by their order in the migrate scanner")

What I don't understand is that "the samll race window" mentioned in
comments is which situation. I think before the
isolate_migratepages_block() function is involved, those pageblocks have
been marked MIGRATE_ISOLATE by set_migratetype_isolate() in
start_isolate_page_range(). So the pages of those pageblocks in buddy
will not be allocated, then the buddy_order_unsafe() here will get a
certainly correct order value.

Could you please tell me what situation it will race with? :)



Thanks,
Yanfei