linux-kernel - Re: [External] Re: [PATCH v3 09/21] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page

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Message-ID: <20201110094830.GA25373@linux>
Date:   Tue, 10 Nov 2020 10:48:34 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        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>,
        Michal Hocko <mhocko@...e.com>,
        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 v3 09/21] mm/hugetlb: Free the vmemmap
 pages associated with each hugetlb page

On Tue, Nov 10, 2020 at 02:40:54PM +0800, Muchun Song wrote:
> Only the first HugeTLB page should split the PMD to PTE. The other 63
> HugeTLB pages
> do not need to split. Here I want to make sure we are the first.

I think terminology is loosing me here.

Say you allocate a 2MB HugeTLB page at ffffea0004100000.

The vmemmap range that the represents this is ffffea0004000000 - ffffea0004200000.
That is a 2MB chunk PMD-mapped.
So, in order to free some of those vmemmap pages, we need to break down
that area, remapping it to PTE-based.
I know what you mean, but we are not really splitting hugetlg pages, but
the memmap range they are represented with.

About:

"Only the first HugeTLB page should split the PMD to PTE. The other 63
HugeTLB pages
do not need to split. Here I want to make sure we are the first."

That only refers to gigantic pages, right?

> > > +static void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > > +{
> > > +     pmd_t *pmd;
> > > +     spinlock_t *ptl;
> > > +     LIST_HEAD(free_pages);
> > > +
> > > +     if (!free_vmemmap_pages_per_hpage(h))
> > > +             return;
> > > +
> > > +     pmd = vmemmap_to_pmd(head);
> > > +     ptl = vmemmap_pmd_lock(pmd);
> > > +     if (vmemmap_pmd_huge(pmd)) {
> > > +             VM_BUG_ON(!pgtable_pages_to_prealloc_per_hpage(h));
> >
> > I think that checking for free_vmemmap_pages_per_hpage is enough.
> > In the end, pgtable_pages_to_prealloc_per_hpage uses free_vmemmap_pages_per_hpage.
> 
> The free_vmemmap_pages_per_hpage is not enough. See the comments above.

My comment was about the VM_BUG_ON.


-- 
Oscar Salvador
SUSE L3