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Subject: Re: Re: [PATCH v2] mm/hugetlb: Defer freeing of huge pages if in
 non-task context
To:     Michal Hocko <mhocko@kernel.org>, Waiman Long <longman@redhat.com>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Matthew Wilcox <willy@infradead.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Andi Kleen <ak@linux.intel.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
References: <20191217012508.31495-1-longman@redhat.com>
 <20191217093143.GC31063@dhcp22.suse.cz>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Tue, 17 Dec 2019 13:50:15 +0300
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On 17.12.2019 12:31, Michal Hocko wrote:
> On Mon 16-12-19 20:25:08, Waiman Long wrote:
> [...]
>> Both the hugetbl_lock and the subpool lock can be acquired in
>> free_huge_page(). One way to solve the problem is to make both locks
>> irq-safe.
> 
> Please document why we do not take this, quite natural path and instead
> we have to come up with an elaborate way instead. I believe the primary
> motivation is that some operations under those locks are quite
> expensive. Please add that to the changelog and ideally to the code as
> well. We probably want to fix those anyway and then this would be a
> temporary workaround.
> 
>> Another alternative is to defer the freeing to a workqueue job.
>>
>> This patch implements the deferred freeing by adding a
>> free_hpage_workfn() work function to do the actual freeing. The
>> free_huge_page() call in a non-task context saves the page to be freed
>> in the hpage_freelist linked list in a lockless manner.
> 
> Do we need to over complicate this (presumably) rare event by a lockless
> algorithm? Why cannot we use a dedicated spin lock for for the linked
> list manipulation? This should be really a trivial code without an
> additional burden of all the lockless subtleties.

Why not llist_add()/llist_del_all() ?