Re: [PATCH v2] mm/memory_hotplug: drain per-cpu pages again during memory offline

[Michal Hocko]

On Thu 03-09-20 12:31:36, Andrew Morton wrote:
> On Thu, 3 Sep 2020 19:36:26 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:
> 
> > (still on vacation, back next week on Tuesday)
> > 
> > I didn't look into discussions in v1, but to me this looks like we are
> > trying to hide an actual bug by implementing hacks in the caller
> > (repeated calls to drain_all_pages()). What about alloc_contig_range()
> > users - you get more allocation errors just because PCP code doesn't
> > play along.
> > 
> > There *is* strong synchronization with the page allocator - however,
> > there seems to be one corner case race where we allow to allocate pages
> > from isolated pageblocks.
> > 
> > I want that fixed instead if possible, otherwise this is just an ugly
> > hack to make the obvious symptoms (offlining looping forever) disappear.
> > 
> > If that is not possible easily, I'd much rather want to see all
> > drain_all_pages() calls being moved to the caller and have the expected
> > behavior documented instead of specifying "there is no strong
> > synchronization with the page allocator" - which is wrong in all but PCP
> > cases (and there only in one possible race?).
> > 
> 
> It's a two-line hack which fixes a bug in -stable kernels, so I'm
> inclined to proceed with it anyway.  We can undo it later on as part of
> a better fix, OK?

Agreed. http://lkml.kernel.org/r/20200904070235.GA15277@xxxxxxxxxxxxxx
for reference.
-- 
Michal Hocko
SUSE Labs