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

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Date:   Wed, 10 Mar 2021 10:56:08 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...e.com>,
        Muchun Song <songmuchun@...edance.com>
Cc:     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, paulmck@...nel.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 3/10/21 7:19 AM, Michal Hocko wrote:
> On Mon 08-03-21 18:28:02, Muchun Song wrote:
> [...]
>> @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
>>  	/*
>>  	 * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
>>  	 */
>> -	if (!in_task()) {
>> +	if (in_atomic()) {
> 
> As I've said elsewhere in_atomic doesn't work for CONFIG_PREEMPT_COUNT=n.
> We need this change for other reasons and so it would be better to pull
> it out into a separate patch which also makes HUGETLB depend on
> PREEMPT_COUNT.

Yes, the issue of calling put_page for hugetlb pages from any context
still needs work.  IMO, that is outside the scope of this series.  We
already have code in this path which blocks/sleeps.

Making HUGETLB depend on PREEMPT_COUNT is too restrictive.  IIUC,
PREEMPT_COUNT will only be enabled if we enable:
PREEMPT "Preemptible Kernel (Low-Latency Desktop)"
PREEMPT_RT "Fully Preemptible Kernel (Real-Time)"
or, other 'debug' options.  These are not enabled in 'more common'
kernels.  Of course, we do not want to disable HUGETLB in common
configurations.

I'll put together a separate patch where we can discuss the merits of
making the change from !in_task to in_atomic, and what work remains in
this put_page area.
-- 
Mike Kravetz