linux-kernel - Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

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Date:   Thu, 11 Mar 2021 09:59:53 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...e.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Muchun Song <songmuchun@...edance.com>, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, dave.hansen@...ux.intel.com, luto@...nel.org,
        peterz@...radead.org, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        osalvador@...e.de, song.bao.hua@...ilicon.com, david@...hat.com,
        naoya.horiguchi@....com, joao.m.martins@...cle.com,
        duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Chen Huang <chenhuang5@...wei.com>,
        Bodeddula Balasubramaniam <bodeddub@...zon.com>
Subject: Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated
 with each HugeTLB page

On 3/11/21 4:17 AM, Michal Hocko wrote:
>> Yeah per cpu preempt counting shouldn't be noticeable but I have to
>> confess I haven't benchmarked it.
> 
> But all this seems moot now http://lkml.kernel.org/r/YEoA08n60+jzsnAl@hirez.programming.kicks-ass.net
> 

The proper fix for free_huge_page independent of this series would
involve:

- Make hugetlb_lock and subpool lock irq safe
- Hand off freeing to a workque if the freeing could sleep

Today, the only time we can sleep in free_huge_page is for gigantic
pages allocated via cma.  I 'think' the concern about undesirable
user visible side effects in this case is minimal as freeing/allocating
1G pages is not something that is going to happen at a high frequency.
My thinking could be wrong?

Of more concern, is the introduction of this series.  If this feature
is enabled, then ALL free_huge_page requests must be sent to a workqueue.
Any ideas on how to address this?
-- 
Mike Kravetz