linux-kernel - Re: [PATCH v7 6/7] mm: Make alloc_contig

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Message-ID: <YHZ1A8Wi43DcEv98@localhost.localdomain>
Date:   Wed, 14 Apr 2021 06:52:19 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 6/7] mm: Make alloc_contig_range handle in-use hugetlb
 pages

On Tue, Apr 13, 2021 at 03:48:53PM -0700, Mike Kravetz wrote:
> The label free_new is:
> 
> free_new:
>         spin_unlock_irq(&hugetlb_lock);
>         __free_pages(new_page, huge_page_order(h));
> 
>         return ret;
> 
> So, we are locking and immediately unlocking without any code in
> between.  Usually, I don't like like multiple labels before return.
> However, perhaps we should add another to avoid this unnecessary
> cycle.  On the other hand, this is an uncommon race condition so the
> simple code may be acceptable.

I guess we could have something like:

 free_new:
         spin_unlock_irq(&hugetlb_lock);
 free_new_nolock:
         __free_pages(new_page, huge_page_order(h));
 
         return ret;

And let the retry go to there without locking. But as you said, the
racecondition is rare enough, so I am not sure if this buys us much.
But I can certainly add it if you feel strong about it.


-- 
Oscar Salvador
SUSE L3