linux-kernel - Re: [PATCH v4 6/8] hugetlb: change free_pool_huge_page to remove_pool_huge

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Date:   Wed, 7 Apr 2021 10:44:38 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Rientjes <rientjes@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        HORIGUCHI NAOYA <naoya.horiguchi@....com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Waiman Long <longman@...hat.com>, Peter Xu <peterx@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Hillf Danton <hdanton@...a.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 6/8] hugetlb: change free_pool_huge_page to
 remove_pool_huge_page

On Mon, Apr 05, 2021 at 04:00:41PM -0700, Mike Kravetz wrote:
> free_pool_huge_page was called with hugetlb_lock held.  It would remove
> a hugetlb page, and then free the corresponding pages to the lower level
> allocators such as buddy.  free_pool_huge_page was called in a loop to
> remove hugetlb pages and these loops could hold the hugetlb_lock for a
> considerable time.
> 
> Create new routine remove_pool_huge_page to replace free_pool_huge_page.
> remove_pool_huge_page will remove the hugetlb page, and it must be
> called with the hugetlb_lock held.  It will return the removed page and
> it is the responsibility of the caller to free the page to the lower
> level allocators.  The hugetlb_lock is dropped before freeing to these
> allocators which results in shorter lock hold times.
> 
> Add new helper routine to call update_and_free_page for a list of pages.
> 
> Note: Some changes to the routine return_unused_surplus_pages are in
> need of explanation.  Commit e5bbc8a6c992 ("mm/hugetlb.c: fix reservation
> race when freeing surplus pages") modified this routine to address a
> race which could occur when dropping the hugetlb_lock in the loop that
> removes pool pages.  Accounting changes introduced in that commit were
> subtle and took some thought to understand.  This commit removes the
> cond_resched_lock() and the potential race.  Therefore, remove the
> subtle code and restore the more straight forward accounting effectively
> reverting the commit.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> Reviewed-by: Muchun Song <songmuchun@...edance.com>
> Acked-by: Michal Hocko <mhocko@...e.com>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

It crossed my mind that we could use update_and_free_pages_bulk() from
dissolve_free_huge_pages() <-> dissolve_free_huge_page() but the latter
looks too hairy already and it is probably not worth it.


-- 
Oscar Salvador
SUSE L3