linux-kernel - Re: [PATCH V5] mm, compaction: don't use ALLOC

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Message-ID: <78586900-a5bc-4377-8fb9-f322f2028310@linux.alibaba.com>
Date: Tue, 17 Dec 2024 10:35:24 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: yangge1116@....com, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 21cnbao@...il.com, david@...hat.com, vbabka@...e.cz, liuzixing@...on.cn
Subject: Re: [PATCH V5] mm, compaction: don't use ALLOC_CMA in long term GUP
 flow



On 2024/12/16 19:54, yangge1116@....com wrote:
> From: yangge <yangge1116@....com>
> 
> Since commit 984fdba6a32e ("mm, compaction: use proper alloc_flags
> in __compaction_suitable()") allow compaction to proceed when free
> pages required for compaction reside in the CMA pageblocks, it's
> possible that __compaction_suitable() always returns true, and in
> some cases, it's not acceptable.
> 
> 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. Since there is 16G of free CMA memory on the NUMA
> node, watermark for order-0 always be met for compaction, so
> __compaction_suitable() always returns true, even if the node is
> unable to allocate non-CMA memory for the virtual machine.
> 
> 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
> 
> In order to quickly fall back to remote node, we should remove
> ALLOC_CMA both in __compaction_suitable() and __isolate_free_page()
> in long term GUP flow. After this fix, starting a 32GB virtual machine
> with device passthrough takes only a few seconds.
> 
> Fixes: 984fdba6a32e ("mm, compaction: use proper alloc_flags in __compaction_suitable()")
> Cc: <stable@...r.kernel.org>
> Signed-off-by: yangge <yangge1116@....com>
> ---

I sent a follow-up fix patch[1] to update the cc->alloc_flags, and with 
that, looks good to me.
Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>

[1] 
https://lore.kernel.org/all/20241217022955.141818-1-baolin.wang@linux.alibaba.com/