Re: [PATCH V2] mm: compaction: use the actual allocation context to determine the watermarks for costly order during async memory compaction

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jan 16, 2025 at 09:33:34AM +0800, yangge1116@126.com wrote:
> From: yangge <yangge1116@126.com>
> 
> 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.
> 
> 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. There is 16GB of free CMA memory on a NUMA node, which is
> sufficient to pass the order-0 watermark check, causing the
> __compaction_suitable() function to  consistently return true.
> 
> For costly allocations, if the __compaction_suitable() function always
> returns true, it causes the __alloc_pages_slowpath() function to fail
> to exit at the appropriate point. This prevents timely fallback to
> allocating memory on other nodes, ultimately 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
> 
> We could use the real unmovable allocation context to have
> __zone_watermark_unusable_free() subtract CMA pages, and thus we won't
> pass the order-0 check anymore once the non-CMA part is exhausted. There
> is some risk that in some different scenario the compaction could in
> fact migrate pages from the exhausted non-CMA part of the zone to the
> CMA part and succeed, and we'll skip it instead. But only __GFP_NORETRY
> allocations should be affected in the immediate "goto nopage" when
> compaction is skipped, others will attempt with DEF_COMPACT_PRIORITY
> anyway and won't fail without trying to compact-migrate the non-CMA
> pageblocks into CMA pageblocks first, so it should be fine.
> 
> After this fix, it only takes a few tens of seconds to start a 32GB
> virtual machine with device passthrough functionality.
> 
> Link: https://lore.kernel.org/lkml/1736335854-548-1-git-send-email-yangge1116@126.com/
> Signed-off-by: yangge <yangge1116@126.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>