Re: [PATCH V7] mm, compaction: don't use ALLOC_CMA for unmovable allocations

[Ge Yang <yangge1116@126.com>]

在 2024/12/17 23:55, Johannes Weiner 写道:
> Hello Yangge,
> 
> On Tue, Dec 17, 2024 at 07:46:44PM +0800, 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
>>
>> Other unmovable alloctions, like dma_buf, which can be large in a
>> Linux system, are also unable to allocate memory from CMA, and these
>> allocations suffer from the same problems described above. In order
>> to quickly fall back to remote node, we should remove ALLOC_CMA both
>> in __compaction_suitable() and __isolate_free_page() for unmovable
>> alloctions. After this fix, starting a 32GB virtual machine with
>> device passthrough takes only a few seconds.
> 
> The symptom is obviously bad, but I don't understand this fix.
> 
> The reason we do ALLOC_CMA is that, even for unmovable allocations,
> you can create space in non-CMA space by moving migratable pages over
> to CMA space. This is not a property we want to lose. But I also don't
> see how it would interfere with your scenario.

The __alloc_pages_slowpath() function was originally intended to exit at 
place 1, but due to __compaction_suitable() always returning true, it 
results in __alloc_pages_slowpath() exiting at place 2 instead. This 
ultimately leads to a significantly longer execution time for 
__alloc_pages_slowpath().

Call trace:
  __alloc_pages_slowpath
       if (compact_result == COMPACT_SKIPPED ||
          compact_result == COMPACT_DEFERRED)
           goto nopage; // place 1
       __alloc_pages_direct_reclaim() // Reclaim is very expensive
       __alloc_pages_direct_compact()
       if (gfp_mask & __GFP_NORETRY)
           goto nopage; // place 2

Every time memory allocation goes through the above slower process, it 
ultimately leads to significantly longer virtual machine startup times.

> 
> There is the compaction_suitable() check in should_compact_retry(),
> but that only applies when COMPACT_SKIPPED. IOW, it should only happen
> when compaction_suitable() just now returned false. IOW, a race
> condition. Which is why it's also not subject to limited retries.
> 
> What's the exact condition that traps the allocator inside the loop?
The should_compact_retry() function was not executed, and the slow here 
was mainly due to the execution of __alloc_pages_direct_reclaim().