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

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Message-ID: <e153119f-036c-7479-9f27-e8d043ee3d2f@oracle.com>
Date:   Tue, 17 Dec 2019 10:33:01 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>, Waiman Long <longman@...hat.com>
Cc:     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: [PATCH v2] mm/hugetlb: Defer freeing of huge pages if in non-task
 context

On 12/17/19 1:31 AM, 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.

We may have talked in the past about how hugetlbfs locking is not ideal.
However, there are many things in hugetlbfs that are not ideal. :(  The
thought was to avoid making changes unless they showed up as real problems.
Looks like the locking is now a real issue.

I'll start work on restructuring at least this part of the locking.  But,
let's move forward with the deferred freeing approach until that is ready.
-- 
Mike Kravetz