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

[Ge Yang]

在 2024/12/17 23:55, Johannes Weiner 写道:
> Hello Yangge,
>
> On Tue, Dec 17, 2024 at 07:46:44PM +0800, yangge1116@xxxxxxx wrote:
> > From: yangge <yangge1116@xxxxxxx>
> >
> > 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().