Re: [PATCH V5] mm, compaction: don't use ALLOC_CMA in long term GUP flow

[Ge Yang <yangge1116@126.com>]

在 2024/12/17 10:35, Baolin Wang 写道:
> 
> 
> On 2024/12/16 19:54, yangge1116@126.com wrote:
>> From: yangge <yangge1116@126.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@vger.kernel.org>
>> Signed-off-by: yangge <yangge1116@126.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@linux.alibaba.com>
> 
> [1] 
> https://lore.kernel.org/all/20241217022955.141818-1-baolin.wang@linux.alibaba.com/
Thanks