linux-kernel - Re: [External] Re: [PATCH v2 07/19] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page

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Message-ID: <CAMZfGtUcknh5Atms=JGWnM-mNodW6n3PV=8TgA865zMZZK-ZpQ@mail.gmail.com>
Date:   Fri, 30 Oct 2020 10:58:59 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, viro@...iv.linux.org.uk,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v2 07/19] mm/hugetlb: Free the vmemmap
 pages associated with each hugetlb page

On Fri, Oct 30, 2020 at 6:00 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 10/28/20 11:13 PM, Muchun Song wrote:
> > On Thu, Oct 29, 2020 at 7:42 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
> >>
> >> On 10/26/20 7:51 AM, Muchun Song wrote:
> >>> +
> >>> +static inline spinlock_t *vmemmap_pmd_lockptr(pmd_t *pmd)
> >>> +{
> >>> +     static DEFINE_SPINLOCK(pgtable_lock);
> >>> +
> >>> +     return &pgtable_lock;
> >>> +}
> >>
> >> This is just a global lock.  Correct?  And hugetlb specific?
> >
> > Yes, it is a global lock. Originally, I wanted to use the pmd lock(e.g.
> > pmd_lockptr()). But we need to allocate memory for the spinlock and
> > initialize it when ALLOC_SPLIT_PTLOCKS. It may increase the
> > complexity.
> >
> > And I think that here alloc/free hugetlb pages is not a frequent operation.
> > So I finally use a global lock. Maybe it is enough.
> >
> >>
> >> It should be OK as the page table entries for huegtlb pages will not
> >> overlap with other entries.
> >
> > Does "hugetlb specific" mean the pmd lock? or per hugetlb lock?
> > If it is pmd lock, this is fine to me. If not, it may not be enough.
> > Because the lock also guards the splitting of pmd pgtable.
>
> By "hugetlb specific", I was trying to say that only hugetlb code would
> use this lock.  It is not a concern now.  However, there has been talk
> about other code doing something similar to remove struct pages.  If that
> ever happens then we will need a different locking scheme.

Agreed, It is not a concern now :)

>
> Disregard my statement about there being no overlap.  I was confusing
> page tables for huge pages with page tables for mappings mmap entries
> of huge pages.
> --
> Mike Kravetz



-- 
Yours,
Muchun