linux-kernel - Re: [External] Re: [PATCH v12 04/13] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

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Date:   Thu, 14 Jan 2021 21:05:06 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        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>,
        Michal Hocko <mhocko@...e.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....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 v12 04/13] mm/hugetlb: Free the vmemmap
 pages associated with each HugeTLB page

On Thu, Jan 14, 2021 at 7:52 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Thu, Jan 14, 2021 at 06:54:30PM +0800, Muchun Song wrote:
> > I think this approach may be only suitable for generic huge page only.
> > So we can implement it only for huge page.
> >
> > Hi Oscar,
> >
> > What's your opinion about this?
>
> I tried something like:
>
> static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
>                               unsigned long end,
>                               struct vmemmap_remap_walk *walk)
> {
>         pte_t *pte;
>
>         pte = pte_offset_kernel(pmd, addr);
>
>         if (!walk->reuse_page) {
>                 BUG_ON(pte_none(*pte));
>
>                 walk->reuse_page = pte_page(*pte++);
>                 addr = walk->remap_start;
>         }
>
>         for (; addr != end; addr += PAGE_SIZE, pte++) {
>                 BUG_ON(pte_none(*pte));
>
>                 walk->remap_pte(pte, addr, walk);
>         }
> }
>
> void vmemmap_remap_free(unsigned long start, unsigned long end,
>                         unsigned long reuse)
> {
>         LIST_HEAD(vmemmap_pages);
>         struct vmemmap_remap_walk walk = {
>                 .remap_pte      = vmemmap_remap_pte,
>                 .reuse_addr     = reuse,
>                 .remap_start = start,
>                 .vmemmap_pages  = &vmemmap_pages,
>         };
>
>         BUG_ON(start != reuse + PAGE_SIZE);
>
>         vmemmap_remap_range(reuse, end, &walk);
>         free_vmemmap_page_list(&vmemmap_pages);
> }
>
> but it might overcomplicate things and I am not sure it is any better.
> So I am fine with keeping it as is.
> Should another user come in the future, we can always revisit.
> Maybe just add a little comment in vmemmap_pte_range(), explaining while we
> are "+= PAGE_SIZE" for address and I would like to see a comment in
> vmemmap_remap_free why the BUG_ON and more important what it is checking.

OK, I will add some comments to explain why we do this in
vmemmap_remap_free and vmemmap_remap_free. Thanks.

>
> --
> Oscar Salvador
> SUSE L3