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/
