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Subject: Re: [PATCH 06/10] mm: vmscan: demote anon DRAM pages to PMEM node
To:     Keith Busch <kbusch@kernel.org>
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From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <39d8fb56-df60-9382-9b47-59081d823c3c@linux.alibaba.com>
Date:   Tue, 26 Mar 2019 20:41:15 -0700
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On 3/26/19 5:35 PM, Keith Busch wrote:
> On Mon, Mar 25, 2019 at 12:49:21PM -0700, Yang Shi wrote:
>> On 3/24/19 3:20 PM, Keith Busch wrote:
>>> How do these pages eventually get to swap when migration fails? Looks
>>> like that's skipped.
>> Yes, they will be just put back to LRU. Actually, I don't expect it would be
>> very often to have migration fail at this stage (but I have no test data to
>> support this hypothesis) since the pages have been isolated from LRU, so
>> other reclaim path should not find them anymore.
>>
>> If it is locked by someone else right before migration, it is likely
>> referenced again, so putting back to LRU sounds not bad.
>>
>> A potential improvement is to have sync migration for kswapd.
> Well, it's not that migration fails only if the page is recently
> referenced. Migration would fail if there isn't available memory in
> the migration node, so this implementation carries an expectation that
> migration nodes have higher free capacity than source nodes. And since
> your attempting THP's without ever splitting them, that also requires
> lower fragmentation for a successful migration.

Yes, it is possible. However, migrate_pages() already has logic to 
handle such case. If the target node has not enough space for migrating 
THP in a whole, it would split THP then retry with base pages.

Swapping THP has been optimized to swap in a whole too. It would try to 
add THP into swap cache in a whole, split THP if the attempt fails, then 
add base pages into swap cache.

So, I think we can leave this to migrate_pages() without splitting in 
advance all the time.

Thanks,
Yang

>
> Applications, however, may allocate and pin pages directly out of that
> migration node to the point it does not have so much free capacity or
> physical continuity, so we probably shouldn't assume it's the only way
> to reclaim pages.