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

[Ge Yang]

在 2024/12/17 10:35, Baolin Wang 写道:
> On 2024/12/16 19:54, 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
> >
> > 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: yangge <yangge1116@xxxxxxx>
> > ---
>
> 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@xxxxxxxxxxxxxxxxx>
>
> [1] 
> https://lore.kernel.org/all/20241217022955.141818-1-baolin.wang@xxxxxxxxxxxxxxxxx/
Thanks