linux-kernel - Re: [PATCH 1/2] mm: Make alloc_contig

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Date:   Fri, 19 Feb 2021 12:17:11 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: Make alloc_contig_range handle free hugetlb pages

On Fri, Feb 19, 2021 at 11:55:00AM +0100, Michal Hocko wrote:
> It is not the lock that I care about but more about counters. The
> intention was that there is a single place to handle both enqueing and
> dequeing. As not all places require counters to be updated. E.g. the
> migration which just replaces one page by another.

I see.
alloc_fresh_huge_page->prep_new_huge_page increments h->nr_huge_pages{_node}
counters.
Which means:

>       new_page = alloc_fresh_huge_page();
>       if (!new_page)
>               goto fail;
>       spin_lock(hugetlb_lock);
>       if (!PageHuge(old_page)) {
>               /* freed from under us, nothing to do */ 
>               __update_and_free_page(new_page);

Here we need update_and_free_page, otherwise we would be leaving a stale value
in h->nr_huge_pages{_node}. 

>               goto unlock;
>       }
>       list_del(&old_page->lru);
>       __update_and_free_page(old_page);

Same here.

>       __enqueue_huge_page(new_page);

This is ok since h->free_huge_pages{_node} do not need to be updated.

-- 
Oscar Salvador
SUSE L3