Re: [PATCH V2 0/6] mm: page_alloc: freelist migratetype hygiene

[Zi Yan]

On 21 Sep 2023, at 6:19, David Hildenbrand wrote:

> On 21.09.23 04:31, Zi Yan wrote:
>> On 20 Sep 2023, at 13:23, Zi Yan wrote:
>>
>>> On 20 Sep 2023, at 12:04, Johannes Weiner wrote:
>>>
>>>> On Wed, Sep 20, 2023 at 09:48:12AM -0400, Johannes Weiner wrote:
>>>>> On Wed, Sep 20, 2023 at 08:07:53AM +0200, Vlastimil Babka wrote:
>>>>>> On 9/20/23 03:38, Zi Yan wrote:
>>>>>>> On 19 Sep 2023, at 20:32, Mike Kravetz wrote:
>>>>>>>
>>>>>>>> On 09/19/23 16:57, Zi Yan wrote:
>>>>>>>>> On 19 Sep 2023, at 14:47, Mike Kravetz wrote:
>>>>>>>>>
>>>>>>>>>> 	--- a/mm/page_alloc.c
>>>>>>>>>> 	+++ b/mm/page_alloc.c
>>>>>>>>>> 	@@ -1651,8 +1651,13 @@ static bool prep_move_freepages_block(struct zone *zone, struct page *page,
>>>>>>>>>>   		end = pageblock_end_pfn(pfn) - 1;
>>>>>>>>>>
>>>>>>>>>>   		/* Do not cross zone boundaries */
>>>>>>>>>> 	+#if 0
>>>>>>>>>>   		if (!zone_spans_pfn(zone, start))
>>>>>>>>>> 			start = zone->zone_start_pfn;
>>>>>>>>>> 	+#else
>>>>>>>>>> 	+	if (!zone_spans_pfn(zone, start))
>>>>>>>>>> 	+		start = pfn;
>>>>>>>>>> 	+#endif
>>>>>>>>>> 	 	if (!zone_spans_pfn(zone, end))
>>>>>>>>>> 	 		return false;
>>>>>>>>>> 	I can still trigger warnings.
>>>>>>>>>
>>>>>>>>> OK. One thing to note is that the page type in the warning changed from
>>>>>>>>> 5 (MIGRATE_ISOLATE) to 0 (MIGRATE_UNMOVABLE) with my suggested change.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Just to be really clear,
>>>>>>>> - the 5 (MIGRATE_ISOLATE) warning was from the __alloc_pages call path.
>>>>>>>> - the 0 (MIGRATE_UNMOVABLE) as above was from the alloc_contig_range call
>>>>>>>>    path WITHOUT your change.
>>>>>>>>
>>>>>>>> I am guessing the difference here has more to do with the allocation path?
>>>>>>>>
>>>>>>>> I went back and reran focusing on the specific migrate type.
>>>>>>>> Without your patch, and coming from the alloc_contig_range call path,
>>>>>>>> I got two warnings of 'page type is 0, passed migratetype is 1' as above.
>>>>>>>> With your patch I got one 'page type is 0, passed migratetype is 1'
>>>>>>>> warning and one 'page type is 1, passed migratetype is 0' warning.
>>>>>>>>
>>>>>>>> I could be wrong, but I do not think your patch changes things.
>>>>>>>
>>>>>>> Got it. Thanks for the clarification.
>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> One idea about recreating the issue is that it may have to do with size
>>>>>>>>>> of my VM (16G) and the requested allocation sizes 4G.  However, I tried
>>>>>>>>>> to really stress the allocations by increasing the number of hugetlb
>>>>>>>>>> pages requested and that did not help.  I also noticed that I only seem
>>>>>>>>>> to get two warnings and then they stop, even if I continue to run the
>>>>>>>>>> script.
>>>>>>>>>>
>>>>>>>>>> Zi asked about my config, so it is attached.
>>>>>>>>>
>>>>>>>>> With your config, I still have no luck reproducing the issue. I will keep
>>>>>>>>> trying. Thanks.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Perhaps try running both scripts in parallel?
>>>>>>>
>>>>>>> Yes. It seems to do the trick.
>>>>>>>
>>>>>>>> Adjust the number of hugetlb pages allocated to equal 25% of memory?
>>>>>>>
>>>>>>> I am able to reproduce it with the script below:
>>>>>>>
>>>>>>> while true; do
>>>>>>>   echo 4 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages&
>>>>>>>   echo 2048 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages&
>>>>>>>   wait
>>>>>>>   echo 0 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
>>>>>>>   echo 0 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>>>>>>> done
>>>>>>>
>>>>>>> I will look into the issue.
>>>>>
>>>>> Nice!
>>>>>
>>>>> I managed to reproduce it ONCE, triggering it not even a second after
>>>>> starting the script. But I can't seem to do it twice, even after
>>>>> several reboots and letting it run for minutes.
>>>>
>>>> I managed to reproduce it reliably by cutting the nr_hugepages
>>>> parameters respectively in half.
>>>>
>>>> The one that triggers for me is always MIGRATE_ISOLATE. With some
>>>> printk-tracing, the scenario seems to be this:
>>>>
>>>> #0                                                   #1
>>>> start_isolate_page_range()
>>>>    isolate_single_pageblock()
>>>>      set_migratetype_isolate(tail)
>>>>        lock zone->lock
>>>>        move_freepages_block(tail) // nop
>>>>        set_pageblock_migratetype(tail)
>>>>        unlock zone->lock
>>>>                                                       del_page_from_freelist(head)
>>>>                                                       expand(head, head_mt)
>>>>                                                         WARN(head_mt != tail_mt)
>>>>      start_pfn = ALIGN_DOWN(MAX_ORDER_NR_PAGES)
>>>>      for (pfn = start_pfn, pfn < end_pfn)
>>>>        if (PageBuddy())
>>>>          split_free_page(head)
>>>>
>>>> IOW, we update a pageblock that isn't MAX_ORDER aligned, then drop the
>>>> lock. The move_freepages_block() does nothing because the PageBuddy()
>>>> is set on the pageblock to the left. Once we drop the lock, the buddy
>>>> gets allocated and the expand() puts things on the wrong list. The
>>>> splitting code that handles MAX_ORDER blocks runs *after* the tail
>>>> type is set and the lock has been dropped, so it's too late.
>>>
>>> Yes, this is the issue I can confirm as well. But it is intentional to enable
>>> allocating a contiguous range at pageblock granularity instead of MAX_ORDER
>>> granularity. With your changes below, it no longer works, because if there
>>> is an unmovable page in
>>> [ALIGN_DOWN(start_pfn, MAX_ORDER_NR_PAGES), pageblock_start_pfn(start_pfn)),
>>> the allocation fails but it would succeed in current implementation.
>>>
>>> I think a proper fix would be to make move_freepages_block() split the
>>> MAX_ORDER page and put the split pages in the right migratetype free lists.
>>>
>>> I am working on that.
>>
>> After spending half a day on this, I think it is much harder than I thought
>> to get alloc_contig_range() working with the freelist migratetype hygiene
>> patchset. Because alloc_contig_range() relies on racy migratetype changes:
>>
>> 1. pageblocks in the range are first marked as MIGRATE_ISOLATE to prevent
>> another parallel isolation, but they are not moved to the MIGRATE_ISOLATE
>> free list yet.
>>
>> 2. later in the process, isolate_freepages_range() is used to actually grab
>> the free pages.
>>
>> 3. there was no problem when alloc_contig_range() works on MAX_ORDER aligned
>> ranges, since MIGRATE_ISOLATE cannot be set in the middle of free pages or
>> in-use pages. But it is not the case when alloc_contig_range() work on
>> pageblock aligned ranges. Now during isolation phase, free or in-use pages
>> will need to be split to get their subpages into the right free lists.
>>
>> 4. the hardest case is when a in-use page sits across two pageblocks, currently,
>> the code just isolate one pageblock, migrate the page, and let split_free_page()
>> to correct the free list later. But to strictly enforce freelist migratetype
>> hygiene, extra work is needed at free page path to split the free page into
>> the right freelists.
>>
>> I need more time to think about how to get alloc_contig_range() properly.
>> Help is needed for the bullet point 4.
>
>
> I once raised that we should maybe try making MIGRATE_ISOLATE a flag that preserves the original migratetype. Not sure if that would help here in any way.

I have that in my backlog since you asked and have been delaying it. ;) Hopefully
I can do it after I fix this. That change might or might not help only if we make
some redesign on how migratetype is managed. If MIGRATE_ISOLATE does not
overwrite existing migratetype, the code might not need to split a page and move
it to MIGRATE_ISOLATE freelist?

The fundamental issue in alloc_contig_range() is that to work at
pageblock level, a page (>pageblock_order) can have one part is isolated and
the rest is a different migratetype. {add_to,move_to,del_page_from}_free_list()
now checks first pageblock migratetype, so such a page needs to be removed
from its free_list, set MIGRATE_ISOLATE on one of the pageblock, split, and
finally put back to multiple free lists. This needs to be done at isolation stage
before free pages are removed from their free lists (the stage after isolation).
If MIGRATE_ISOLATE is a separate flag and we are OK with leaving isolated pages
in their original migratetype and check migratetype before allocating a page,
that might help. But that might add extra work (e.g., splitting a partially
isolated free page before allocation) in the really hot code path, which is not
desirable.

>
> The whole alloc_contig_range() implementation is quite complicated and hard to grasp. If we could find ways to clean all that up and make it easier to understand and play along, that would be nice.

I will try my best to simplify it.

--
Best Regards,
Yan, Zi

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