Re: [PATCH 00/10] fix freepage count problems due to memory isolation

[Joonsoo Kim]

Ccing Lisa, because there was bug report it may be related this
topic last Saturday.

http://www.spinics.net/lists/linux-mm/msg75741.html

On Fri, Jul 04, 2014 at 05:33:27PM +0200, Vlastimil Babka wrote:
> On 07/04/2014 09:57 AM, Joonsoo Kim wrote:
> > Hello,
> 
> Hi Joonsoo,
> 
> please CC me on further updates, this is relevant to me.

Hello, Vlastimil.

Sorry for missing you. :)

> 
> > This patchset aims at fixing problems due to memory isolation found by
> > testing my patchset [1].
> > 
> > These are really subtle problems so I can be wrong. If you find what I am
> > missing, please let me know.
> > 
> > Before describing bugs itself, I first explain definition of freepage.
> > 
> > 1. pages on buddy list are counted as freepage.
> > 2. pages on isolate migratetype buddy list are *not* counted as freepage.
> 
> I think the second point is causing us a lot of trouble. And I wonder if it's really
> justified! We already have some is_migrate_isolate() checks in the fast path and now you
> would add more, mostly just to keep this accounting correct.

It's not just for keeping accouting correct. There is another
purpose for is_migrate_isolate(). It forces freed pages to go into
isolate buddy list. Without it, freed pages would go into other
buddy list and will be used soon. So memory isolation can't work well
without is_migrate_isolate() checks and success rate could decrease.

And, I just added three more is_migrate_isolate() in the fast
path, but, two checks are in same *unlikely* branch and I can remove
another one easily. Therefore it's not quite problem I guess. (It even
does no-op if MEMORY_ISOLATION is disabled.)
Moreover, I removed one unconditional get_pageblock_migratetype() in
free_pcppages_bulk() so, in performance point or view, freepath would
be improved.

> 
> So the question is, does it have to be correct? And (admiteddly not after a completely
> exhaustive analysis) I think the answer is, surprisingly, that it doesn't :)
> 
> Well I of course don't mean that the freepage accounts could go random completely, but
> what if we allowed them to drift a bit, limiting both the max error and the timeframe
> where errors are possible? After all, watermarks checking is already racy so I don't think
> it would be hurt that much.

I understand your suggestion. I once thought like as you, but give up
that idea. Watermark checking is already racy, but, it's *only*
protection to prevent memory allocation. After passing that check,
there is no mean to prevent us from allocating memory. So it should
be accurate as much as possible. If we allow someone to get the
memory without considering memory isolation, free memory could be in
really low state and system could be broken occasionally.

> 
> Now if we look at what both CMA and memory hot-remove does is:
> 
> 1. Mark a MAX_ORDER-aligned buch of pageblocks as MIGRATE_ISOLATE through
> start_isolate_page_range(). As part of this, all free pages in that area are
> moved on the isolate freelist through move_freepages_block().
> 
> 2. Try to migrate away all non-free pages in the range. Also drain pcplists and lru_add
> caches.
> 
> 3. Check if everything was successfully isolated by test_pages_isolated(). Restart and/or
> undo pageblock isolation if not.
> 
> So my idea is to throw away all special-casing of is_migrate_isolate() in the buddy
> allocator, which would therefore account free pages on the isolate freelist as normal free
> pages.
> The accounting of isolated pages would be instead done only on the top level of CMA/hot
> remove in the three steps outlined above, which would be modified as follows:
> 
> 1. Calculate N as the target number of pages to be isolated. Perform the actions of step 1
> as usual. Calculate X as the number of pages that move_freepages_block() has moved.
> Subtract X from freepages (this is the same as it is done now), but also *remember the
> value of X*
> 
> 2. Migrate and drain pcplists as usual. The new free pages will either end up correctly on
> isolate freelist, or not. We don't care, they will be accounted as freepages either way.
> This is where some inaccuracy in accounted freepages would build up.
> 
> 3. If test_pages_isolated() checks pass, subtract (N - X) from freepages. The result is
> that we have a isolated range of N pages that nobody can steal now as everything is on
> isolate freelist and is MAX_ORDER aligned. And we have in total subtracted N pages (first
> X, then N-X). So the accounting matches reality.
> 
> If we have to undo, we undo the isolation and as part of this, we use
> move_freepages_block() to move pages from isolate freelist to the normal ones. But we
> don't care how many pages were moved. We simply add the remembered value of X to the
> number of freepages, undoing the change from step 1. Again, the accounting matches reality.
> 
> 
> The final point is that if we do this per MAX_ORDER blocks, the error in accounting cannot
> be ever larger than 4MB and will be visible only during time a single MAX_ORDER block is
> handled.

The 4MB error in accounting looks serious for me. min_free_kbytes is
4MB in 1GB system. So this 4MB error would makes all things broken in
such systems. Moreover, there are some ARCH having larger
pageblock_order than MAX_ORDER. In this case, the error will be larger
than 4MB.

In addition, I have a plan to extend CMA to work in parallel. It means
that there could be parallel memory isolation users rather than just
one user at the same time, so, we cannot easily bound the error under
some degree.

> 
> As a possible improvement, we can assume during phase 2 that every page freed by migration
> will end up correctly on isolate free list. So we create M free pages by migration, and
> subtract M from freepage account. Then in phase 3 we either subtract (N - X - M), or add X
> + M in the undo case. (Ideally, if we succeed, X + M should be equal to N, but due to
> pages on pcplists and the possible races it will be less). I think with this improvement,
> any error would be negligible.
> 
> Thoughts?

Thanks for suggestion. :)
It is really good topic to think deeply.

For now, I'd like to fix these problems without side-effect as you
suggested. Your suggestion changes the meaning of freepage that
isolated pages are included in nr_freepage and there could be possible
regression in success rate of memory hotplug and CMA. Possibly, it
is the way we have to go, but, IMHO, it isn't the time to go. Before
going that way, we should fix current implementation first so that
fixes can be backported to old kernel if someone needs. Interestingly,
on last Saturday, Lisa Du reported CMA accounting bugs.

http://www.spinics.net/lists/linux-mm/msg75741.html

I don't look at it in detail, but, maybe it is related to these
problems and we should fix it without side-effect.

So, in conclusion, I think that your suggestion is beyond the scope of
this patchset because of following two reasons.

1. I'd like to fix these problems without side-effect(possible
regression in success rate of memory hotplug and CMA, and nr_freepage
meanging change) due to backport possibility.
2. nr_freepage without considering memory isolation is somewhat dangerous
and not suitable for some systems.

If you have any objection, please let me know. But, I will go on
a vacation for a week so I can't answer your further comments
for a week. I will reply them next week. :)

Thanks.

> > 3. pages on cma buddy list are counted as CMA freepage, too.
> > 4. pages for guard are *not* counted as freepage.
> > 
> > Now, I describe problems and related patch.
> > 
> > 1. Patch 2: If guard page are cleared and merged into isolate buddy list,
> > we should not add freepage count.
> > 
> > 2. Patch 3: When the page return back from pcp to buddy, we should
> > account it to freepage counter. In this case, we should check the
> > pageblock migratetype of the page and should insert the page into
> > appropriate buddy list. Although we checked it in current code, we
> > didn't insert the page into appropriate buddy list so that freepage
> > counting can be wrong.
> > 
> > 3. Patch 4: There is race condition so that some freepages could be
> > on isolate buddy list. If so, we can't use this page until next isolation
> > attempt on this pageblock.
> > 
> > 4. Patch 5: There is race condition that page on isolate pageblock
> > can go into non-isolate buddy list. If so, buddy allocator would
> > merge pages on non-isolate buddy list and isolate buddy list, respectively,
> > and freepage count will be wrong.
> > 
> > 5. Patch 9: move_freepages(_block) returns *not* number of moved pages.
> > Instead, it returns number of pages linked in that migratetype buddy list.
> > So accouting with this return value makes freepage count wrong.
> > 
> > 6. Patch 10: buddy allocator would merge pages on non-isolate buddy list
> > and isolate buddy list, respectively. This leads to freepage counting
> > problem so fix it by stopping merging in this case.
> > 
> > Without patchset [1], above problem doesn't happens on my CMA allocation
> > test, because CMA reserved pages aren't used at all. So there is no
> > chance for above race.
> > 
> > With patchset [1], I did simple CMA allocation test and get below result.
> > 
> > - Virtual machine, 4 cpus, 1024 MB memory, 256 MB CMA reservation
> > - run kernel build (make -j16) on background
> > - 30 times CMA allocation(8MB * 30 = 240MB) attempts in 5 sec interval
> > - Result: more than 5000 freepage count are missed
> > 
> > With patchset [1] and this patchset, I found that no freepage count are
> > missed so that I conclude that problems are solved.
> > 
> > These problems can be possible on memory hot remove users, although
> > I didn't check it further.
> > 
> > Other patches are either for the base to fix these problems or for
> > simple clean-up. Please see individual patches for more information.
> > 
> > This patchset is based on linux-next-20140703.
> > 
> > Thanks.
> > 
> > [1]: Aggressively allocate the pages on cma reserved memory
> >      https://lkml.org/lkml/2014/5/30/291
> > 
> > 
> > Joonsoo Kim (10):
> >   mm/page_alloc: remove unlikely macro on free_one_page()
> >   mm/page_alloc: correct to clear guard attribute in DEBUG_PAGEALLOC
> >   mm/page_alloc: handle page on pcp correctly if it's pageblock is
> >     isolated
> >   mm/page_alloc: carefully free the page on isolate pageblock
> >   mm/page_alloc: optimize and unify pageblock migratetype check in free
> >     path
> >   mm/page_alloc: separate freepage migratetype interface
> >   mm/page_alloc: store migratetype of the buddy list into freepage
> >     correctly
> >   mm/page_alloc: use get_onbuddy_migratetype() to get buddy list type
> >   mm/page_alloc: fix possible wrongly calculated freepage counter
> >   mm/page_alloc: Stop merging pages on non-isolate and isolate buddy
> >     list
> > 
> >  include/linux/mm.h             |   30 +++++++--
> >  include/linux/mmzone.h         |    5 ++
> >  include/linux/page-isolation.h |    8 +++
> >  mm/page_alloc.c                |  138 +++++++++++++++++++++++++++++-----------
> >  mm/page_isolation.c            |   18 ++----
> >  5 files changed, 147 insertions(+), 52 deletions(-)
> > 
> 
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