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From:   Zi Yan <ziy@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH V2 0/6] mm: page_alloc: freelist migratetype hygiene
Date:   Thu, 21 Sep 2023 10:47:23 -0400
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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 =3D pageblock_end_pfn(pfn) - 1;
>>>>>>>>>>
>>>>>>>>>>   		/* Do not cross zone boundaries */
>>>>>>>>>> 	+#if 0
>>>>>>>>>>   		if (!zone_spans_pfn(zone, start))
>>>>>>>>>> 			start =3D zone->zone_start_pfn;
>>>>>>>>>> 	+#else
>>>>>>>>>> 	+	if (!zone_spans_pfn(zone, start))
>>>>>>>>>> 	+		start =3D 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 chan=
ged 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 cal=
l path.
>>>>>>>> - the 0 (MIGRATE_UNMOVABLE) as above was from the alloc_contig_r=
ange call
>>>>>>>>    path WITHOUT your change.
>>>>>>>>
>>>>>>>> I am guessing the difference here has more to do with the alloca=
tion 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' warnin=
g.
>>>>>>>>
>>>>>>>> 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 h=
ugetlb
>>>>>>>>>> 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 mem=
ory?
>>>>>>>
>>>>>>> I am able to reproduce it with the script below:
>>>>>>>
>>>>>>> while true; do
>>>>>>>   echo 4 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepa=
ges&
>>>>>>>   echo 2048 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepa=
ges&
>>>>>>>   wait
>>>>>>>   echo 0 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages=

>>>>>>>   echo 0 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepa=
ges
>>>>>>> done
>>>>>>>
>>>>>>> I will look into the issue.
>>>>>
>>>>> Nice!
>>>>>
>>>>> I managed to reproduce it ONCE, triggering it not even a second aft=
er
>>>>> 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, h=
ead_mt)
>>>>                                                         WARN(head_mt=
 !=3D tail_mt)
>>>>      start_pfn =3D ALIGN_DOWN(MAX_ORDER_NR_PAGES)
>>>>      for (pfn =3D start_pfn, pfn < end_pfn)
>>>>        if (PageBuddy())
>>>>          split_free_page(head)
>>>>
>>>> IOW, we update a pageblock that isn't MAX_ORDER aligned, then drop t=
he
>>>> lock. The move_freepages_block() does nothing because the PageBuddy(=
)
>>>> is set on the pageblock to the left. Once we drop the lock, the budd=
y
>>>> 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 t=
o 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 th=
e
>>> 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 th=
ought
>> to get alloc_contig_range() working with the freelist migratetype hygi=
ene
>> patchset. Because alloc_contig_range() relies on racy migratetype chan=
ges:
>>
>> 1. pageblocks in the range are first marked as MIGRATE_ISOLATE to prev=
ent
>> another parallel isolation, but they are not moved to the MIGRATE_ISOL=
ATE
>> 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 a=
ligned
>> ranges, since MIGRATE_ISOLATE cannot be set in the middle of free page=
s 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 p=
ages
>> 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_f=
ree_page()
>> to correct the free list later. But to strictly enforce freelist migra=
tetype
>> 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() proper=
ly.
>> Help is needed for the bullet point 4.
>
>
> I once raised that we should maybe try making MIGRATE_ISOLATE a flag th=
at preserves the original migratetype. Not sure if that would help here i=
n any way.

I have that in my backlog since you asked and have been delaying it. ;) H=
opefully
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 a=
nd 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 remove=
d
from its free_list, set MIGRATE_ISOLATE on one of the pageblock, split, a=
nd
finally put back to multiple free lists. This needs to be done at isolati=
on stage
before free pages are removed from their free lists (the stage after isol=
ation).
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 p=
age,
that might help. But that might add extra work (e.g., splitting a partial=
ly
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 eas=
ier 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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