Re: freepage accounting bug with CMA/migrate isolation

[Vlastimil Babka <vbabka@suse.cz>]

On 07/26/2018 06:50 PM, Mike Kravetz wrote:
> On 07/26/2018 05:28 AM, Vlastimil Babka wrote:
>>> Just looking for suggesting in where/how to debug.  I've been hacking on
>>> this without much success.
> 
> As mentioned in my reply to Laura, I noticed that move_freepages_block()
> can move more than a pageblock of pages.  This is the case where page_order
> of the (first) free page is > pageblock_order.  Should only happen in the
> set_migratetype_isolate case as unset has that check you added.  Thi

Hmm not sure which "check I added" you mean, in
unset_migratetype_isolate() ?

> generally 'works' as alloc_contig_range rounds up to MAX_ORDER(-1).  So,
> set and unset migrate isolate tend to balance out.  But, I am wondering
> if there might be some kind of race where someone could mess with those
> pageblocks (and freepage counts) while we drop the zone lock.  Trying to

Yeah see my other mail for such race when we drop the zone lock in
unset_migratetype_isolate(). set_migratetype_isolate() would also have
this problem (which would result in *less* freepages counted), but if we
move MAX_ORDER-1 pages to MIGRATE_ISOLATE freelist, then nobody can mess
with them while the zone is locked, as they are isolated.
unset_migratetype_isolate() has no such luck.

> put together a quick hack to test this theory, but it is more complicated
> that first thought. :)
>