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SubjectRe: [PATCH v3 2/3] mm: disable LRU pagevec during the migration temporarily
On 2021-03-11 14:41, Chris Goldsworthy wrote:
> On 2021-03-10 08:14, Minchan Kim wrote:
>> LRU pagevec holds refcount of pages until the pagevec are drained.
>> It could prevent migration since the refcount of the page is greater
>> than the expection in migration logic. To mitigate the issue,
>> callers of migrate_pages drains LRU pagevec via migrate_prep or
>> lru_add_drain_all before migrate_pages call.
>>
>> However, it's not enough because pages coming into pagevec after the
>> draining call still could stay at the pagevec so it could keep
>> preventing page migration. Since some callers of migrate_pages have
>> retrial logic with LRU draining, the page would migrate at next trail
>> but it is still fragile in that it doesn't close the fundamental race
>> between upcoming LRU pages into pagvec and migration so the migration
>> failure could cause contiguous memory allocation failure in the end.
>>
>> To close the race, this patch disables lru caches(i.e, pagevec)
>> during ongoing migration until migrate is done.
>>
>> Since it's really hard to reproduce, I measured how many times
>> migrate_pages retried with force mode(it is about a fallback to a
>> sync migration) with below debug code.
>>
>> int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> ..
>> ..
>>
>> if (rc && reason == MR_CONTIG_RANGE && pass > 2) {
>> printk(KERN_ERR, "pfn 0x%lx reason %d\n", page_to_pfn(page),
>> rc);
>> dump_page(page, "fail to migrate");
>> }
>>
>> The test was repeating android apps launching with cma allocation
>> in background every five seconds. Total cma allocation count was
>> about 500 during the testing. With this patch, the dump_page count
>> was reduced from 400 to 30.
>>
>> The new interface is also useful for memory hotplug which currently
>> drains lru pcp caches after each migration failure. This is rather
>> suboptimal as it has to disrupt others running during the operation.
>> With the new interface the operation happens only once. This is also
>> in
>> line with pcp allocator cache which are disabled for the offlining as
>> well.
>>
>> Signed-off-by: Minchan Kim <minchan@kernel.org>
>> ---
>
> Hi Minchan,
>
> This all looks good to me - feel free to add a Reviewed-by from me.
>
> Thanks,
>
> Chris.
Should have added:

Reviewed-by: Chris Goldsworthy <cgoldswo@codeaurora.org>
--
The Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
Forum,
a Linux Foundation Collaborative Project

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