linux-kernel - Re: [PATCH V3] mm: compaction: skip memory compaction when there are not enough migratable pages

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Message-ID: <CAGsJ_4yA04vEOTm3CLJ6EEY65Wbpa-YAnwd2t5mi7wq75P_P4Q@mail.gmail.com>
Date: Mon, 13 Jan 2025 21:47:28 +1300
From: Barry Song <21cnbao@...il.com>
To: yangge1116@....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

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.


> +       /*
> +        * 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