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

[Johannes Weiner <hannes@cmpxchg.org>]

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.

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?