linux-kernel - Re: Re: [PATCH v2] mm/hugetlb: Defer freeing of huge pages if in non-task context

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Date:   Tue, 17 Dec 2019 13:50:15 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Michal Hocko <mhocko@...nel.org>, Waiman Long <longman@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Andi Kleen <ak@...ux.intel.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Subject: Re: Re: [PATCH v2] mm/hugetlb: Defer freeing of huge pages if in
 non-task context

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() ?