linux-kernel - Re: [External] Re: [PATCH v3 05/21] mm/hugetlb: Introduce pgtable allocation/freeing helpers

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Date:   Tue, 10 Nov 2020 11:49:27 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Oscar Salvador <osalvador@...e.de>
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 05/21] mm/hugetlb: Introduce pgtable
 allocation/freeing helpers

On Tue, Nov 10, 2020 at 1:21 AM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Sun, Nov 08, 2020 at 10:10:57PM +0800, Muchun Song wrote:
> > +static inline unsigned int pgtable_pages_to_prealloc_per_hpage(struct hstate *h)
> > +{
> > +     unsigned long vmemmap_size = vmemmap_pages_size_per_hpage(h);
> > +
> > +     /*
> > +      * No need pre-allocate page tabels when there is no vmemmap pages
> > +      * to free.
>  s /tabels/tables/

Thanks.

>
> > +static int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> > +{
> > +     int i;
> > +     pgtable_t pgtable;
> > +     unsigned int nr = pgtable_pages_to_prealloc_per_hpage(h);
> > +
> > +     if (!nr)
> > +             return 0;
> > +
> > +     vmemmap_pgtable_init(page);
> > +
> > +     for (i = 0; i < nr; i++) {
> > +             pte_t *pte_p;
> > +
> > +             pte_p = pte_alloc_one_kernel(&init_mm);
> > +             if (!pte_p)
> > +                     goto out;
> > +             vmemmap_pgtable_deposit(page, virt_to_page(pte_p));
> > +     }
> > +
> > +     return 0;
> > +out:
> > +     while (i-- && (pgtable = vmemmap_pgtable_withdraw(page)))
> > +             pte_free_kernel(&init_mm, page_to_virt(pgtable));
>
>         would not be enough to:
>
>         while (pgtable = vmemmap_pgtable_withdrag(page))
>                 pte_free_kernel(&init_mm, page_to_virt(pgtable));

The vmemmap_pgtable_withdraw can not return NULL. So we can not
drop the "i--".

>
> > +     return -ENOMEM;
> > +}
> > +
> > +static void vmemmap_pgtable_free(struct hstate *h, struct page *page)
> > +{
> > +     pgtable_t pgtable;
> > +     unsigned int nr = pgtable_pages_to_prealloc_per_hpage(h);
> > +
> > +     if (!nr)
> > +             return;
>
> We can get rid of "nr" and its check and keep only the check below, right?

Great, the check can go away.

> AFAICS, they go together, e.g: if page_huge_pte does not return null,
> it means that we preallocated a pagetable, and viceversa.
>
>
> > +
> > +     pgtable = page_huge_pte(page);
> > +     if (!pgtable)
> > +             return;
> > +
> > +     while (nr-- && (pgtable = vmemmap_pgtable_withdraw(page)))
> > +             pte_free_kernel(&init_mm, page_to_virt(pgtable));
>
>         Same as above, that "nr" can go?

Here "nr" can not go. Because the vmemmap_pgtable_withdraw can
not return NULL.

Thanks.

>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun