linux-kernel - Re: [PATCH 0/3] THP Shrinker

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Date:   Wed, 28 Sep 2022 16:22:34 +0000
From:   "Alex Zhu (Kernel)" <alexlzhu@...com>
To:     David Hildenbrand <david@...hat.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Matthew Wilcox <willy@...radead.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "riel@...riel.com" <riel@...riel.com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH 0/3] THP Shrinker

Sorry about that. The diff to the RFC:

-Remove all THPs that are not in the top utilization bucket. This is what we have found to perform the best in production testing, we have found that there are an almost trivial number of THPs in the middle range of buckets that account for most of the memory waste. 

-Added check for THP utilization prior to split_huge_page for the THP Shrinker. This is to account for THPs that move to the top bucket, but were underutilized at the time they were added to the list_lru. 

-Refactored out the code to obtain the thp_utilization_bucket, as that now has to be used in multiple places.

-Multiply the shrink_count and scan_count by HPAGE_PMD_NR. This is because a THP is 512 pages, and should count as 512 objects in reclaim. This way reclaim is triggered at a more appropriate frequency than in the RFC. 

-Added support to map to the read only zero page when splitting a THP registered with userfaultfd. Also added a self test to verify that this is working. 

-Only trigger the unmap_clean/zap in split_huge_page on anonymous THPs. We cannot zap zero pages for file THPs.

Thanks,
Alex