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Message-ID: <b6123b34-44f5-4b8e-b68a-d7bb400b9bf0@126.com>
Date: Mon, 13 Jan 2025 19:23:40 +0800
From: Ge Yang <yangge1116@....com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, david@...hat.com,
baolin.wang@...ux.alibaba.com, hannes@...xchg.org, liuzixing@...on.cn
Subject: Re: [PATCH V3] mm: compaction: skip memory compaction when there are
not enough migratable pages
在 2025/1/13 18:05, Barry Song 写道:
> On Mon, Jan 13, 2025 at 10:04 PM Ge Yang <yangge1116@....com> wrote:
>>
>>
>>
>> 在 2025/1/13 16:47, Barry Song 写道:
>>> On Thu, Jan 9, 2025 at 12:31 AM <yangge1116@....com> wrote:
>>>>
>>>> From: yangge <yangge1116@....com>
>>>>
>>>> There are 4 NUMA nodes on my machine, and each NUMA node has 32GB
>>>> of memory. I have configured 16GB of CMA memory on each NUMA node,
>>>> and starting a 32GB virtual machine with device passthrough is
>>>> extremely slow, taking almost an hour.
>>>>
>>>> During the start-up of the virtual machine, it will call
>>>> pin_user_pages_remote(..., FOLL_LONGTERM, ...) to allocate memory.
>>>> Long term GUP cannot allocate memory from CMA area, so a maximum of
>>>> 16 GB of no-CMA memory on a NUMA node can be used as virtual machine
>>>> memory. There is 16GB of free CMA memory on a NUMA node, which is
>>>> sufficient to pass the order-0 watermark check, causing the
>>>> __compaction_suitable() function to consistently return true.
>>>> However, if there aren't enough migratable pages available, performing
>>>> memory compaction is also meaningless. Besides checking whether
>>>> the order-0 watermark is met, __compaction_suitable() also needs
>>>> to determine whether there are sufficient migratable pages available
>>>> for memory compaction.
>>>>
>>>> For costly allocations, because __compaction_suitable() always
>>>> returns true, __alloc_pages_slowpath() can't exit at the appropriate
>>>> place, resulting in excessively long virtual machine startup times.
>>>> Call trace:
>>>> __alloc_pages_slowpath
>>>> if (compact_result == COMPACT_SKIPPED ||
>>>> compact_result == COMPACT_DEFERRED)
>>>> goto nopage; // should exit __alloc_pages_slowpath() from here
>>>>
>>>> When the 16G of non-CMA memory on a single node is exhausted, we will
>>>> fallback to allocating memory on other nodes. In order to quickly
>>>> fallback to remote nodes, we should skip memory compaction when
>>>> migratable pages are insufficient. After this fix, it only takes a
>>>> few tens of seconds to start a 32GB virtual machine with device
>>>> passthrough functionality.
>>>>
>>>> Signed-off-by: yangge <yangge1116@....com>
>>>> ---
>>>>
>>>> V3:
>>>> - fix build error
>>>>
>>>> V2:
>>>> - consider unevictable folios
>>>>
>>>> mm/compaction.c | 20 ++++++++++++++++++++
>>>> 1 file changed, 20 insertions(+)
>>>>
>>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>>> index 07bd227..a9f1261 100644
>>>> --- a/mm/compaction.c
>>>> +++ b/mm/compaction.c
>>>> @@ -2383,7 +2383,27 @@ static bool __compaction_suitable(struct zone *zone, int order,
>>>> int highest_zoneidx,
>>>> unsigned long wmark_target)
>>>> {
>>>> + pg_data_t __maybe_unused *pgdat = zone->zone_pgdat;
>>>> + unsigned long sum, nr_pinned;
>>>> unsigned long watermark;
>>>> +
>>>> + sum = node_page_state(pgdat, NR_INACTIVE_FILE) +
>>>> + node_page_state(pgdat, NR_INACTIVE_ANON) +
>>>> + node_page_state(pgdat, NR_ACTIVE_FILE) +
>>>> + node_page_state(pgdat, NR_ACTIVE_ANON) +
>>>> + node_page_state(pgdat, NR_UNEVICTABLE);
>>>> +
>>>> + nr_pinned = node_page_state(pgdat, NR_FOLL_PIN_ACQUIRED) -
>>>> + node_page_state(pgdat, NR_FOLL_PIN_RELEASED);
>>>> +
>>>
>>> Does the sum of all LRU pages equal non-CMA memory?
>>> I'm quite confused for two reasons:
>>> 1. CMA pages can be LRU pages.
>>> 2. Free pages might not belong to any LRUs.
>> NO.
>>
>> If all the pages in the LRU are pinned, it seems unnecessary to perform
>> memory compaction, as the migration of pinned pages is unlikely to succeed.
>> Besides checking whether the order-0 watermark is met,
>> __compaction_suitable() also needs to determine whether there are
>> sufficient migratable pages available for memory compaction.
>
> Ok, but I am not convinced that this is a correct patch. If all your
> CMA pages are
> used by userspace—in other words, they are in LRUs—the sum could become
> quite large, and `nr_pinned` might include non-CMA pages. In that case,
> `sum - nr_pinned` would also be quite large. The "return false" logic wouldn't
> work as intended.
>
> I suspect the issue seems to have disappeared simply because your CMA is
> not being used at all.
>
Part of the CMA has been used. Due to __compaction_suitable() always
returning true, it triggers swapping, which evicts the already-used CMA
pages to disk, ultimately resulting in only pinned pages remaining in
the LRU (Least Recently Used) list.
>>>
>>>
>>>> + /*
>>>> + * Gup-pinned pages are non-migratable. After subtracting these pages,
>>>> + * we need to check if the remaining pages are sufficient for memory
>>>> + * compaction.
>>>> + */
>>>> + if ((sum - nr_pinned) < (1 << order))
>>>> + return false;
>>>> +
>>>> /*
>>>> * Watermarks for order-0 must be met for compaction to be able to
>>>> * isolate free pages for migration targets. This means that the
>>>> --
>>>> 2.7.4
>>>>
>>>>
>>>
>
> Thanks
> Barry
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