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Date: Fri, 6 Nov 2020 00:08:22 +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>,
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 05/19] mm/hugetlb: Introduce pgtable
allocation/freeing helpers
On Thu, Nov 5, 2020 at 9:23 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Mon, Oct 26, 2020 at 10:51:00PM +0800, Muchun Song wrote:
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +#define VMEMMAP_HPAGE_SHIFT PMD_SHIFT
> > +#define arch_vmemmap_support_huge_mapping() boot_cpu_has(X86_FEATURE_PSE)
>
> I do not think you need this.
> We already have hugepages_supported().
Maybe some architectures support hugepage, but the vmemmap do not
use the hugepage map. In this case, we need it. But I am not sure if it
exists in the real world. At least, x86 can reuse hugepages_supported.
>
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +#ifndef arch_vmemmap_support_huge_mapping
> > +static inline bool arch_vmemmap_support_huge_mapping(void)
> > +{
> > + return false;
> > +}
>
> Same as above
>
> > static inline unsigned int nr_free_vmemmap(struct hstate *h)
> > {
> > return h->nr_free_vmemmap_pages;
> > }
> >
> > +static inline unsigned int nr_vmemmap(struct hstate *h)
> > +{
> > + return nr_free_vmemmap(h) + RESERVE_VMEMMAP_NR;
> > +}
> > +
> > +static inline unsigned long nr_vmemmap_size(struct hstate *h)
> > +{
> > + return (unsigned long)nr_vmemmap(h) << PAGE_SHIFT;
> > +}
> > +
> > +static inline unsigned int nr_pgtable(struct hstate *h)
> > +{
> > + unsigned long vmemmap_size = nr_vmemmap_size(h);
> > +
> > + if (!arch_vmemmap_support_huge_mapping())
> > + return 0;
> > +
> > + /*
> > + * No need pre-allocate page tabels when there is no vmemmap pages
> > + * to free.
> > + */
> > + if (!nr_free_vmemmap(h))
> > + return 0;
> > +
> > + return ALIGN(vmemmap_size, VMEMMAP_HPAGE_SIZE) >> VMEMMAP_HPAGE_SHIFT;
> > +}
>
> IMHO, Mike's naming suggestion fit much better.
I will do that.
>
> > +static void vmemmap_pgtable_deposit(struct page *page, pte_t *pte_p)
> > +{
> > + pgtable_t pgtable = virt_to_page(pte_p);
> > +
> > + /* FIFO */
> > + if (!page_huge_pte(page))
> > + INIT_LIST_HEAD(&pgtable->lru);
> > + else
> > + list_add(&pgtable->lru, &page_huge_pte(page)->lru);
> > + page_huge_pte(page) = pgtable;
> > +}
>
> I think it would make more sense if this took a pgtable argument
> instead of a pte_t *.
Will do. Thanks for your suggestions.
>
> > +static pte_t *vmemmap_pgtable_withdraw(struct page *page)
> > +{
> > + pgtable_t pgtable;
> > +
> > + /* FIFO */
> > + pgtable = page_huge_pte(page);
> > + if (unlikely(!pgtable))
> > + return NULL;
>
> AFAICS, above check only needs to be run once.
> It think we can move it to vmemmap_pgtable_free, can't we?
Yeah, we can. Thanks.
>
> > + page_huge_pte(page) = list_first_entry_or_null(&pgtable->lru,
> > + struct page, lru);
> > + if (page_huge_pte(page))
> > + list_del(&pgtable->lru);
> > + return page_to_virt(pgtable);
> > +}
>
> At the risk of adding more code, I think it would be nice to return a
> pagetable_t?
> So it is more coherent with the name of the function and with what
> we are doing.
Yeah.
>
> It is a pity we cannot converge these and pgtable_trans_huge_*.
> They share some code but it is different enough.
>
> > +static int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> > +{
> > + int i;
> > + pte_t *pte_p;
> > + unsigned int nr = nr_pgtable(h);
> > +
> > + if (!nr)
> > + return 0;
> > +
> > + vmemmap_pgtable_init(page);
>
> Maybe just open code this one?
Sorry. I don't quite understand what it means. Could you explain?
>
> > +static inline void vmemmap_pgtable_free(struct hstate *h, struct page *page)
> > +{
> > + pte_t *pte_p;
> > +
> > + if (!nr_pgtable(h))
> > + return;
> > +
> > + while ((pte_p = vmemmap_pgtable_withdraw(page)))
> > + pte_free_kernel(&init_mm, pte_p);
>
> As mentioned above, move the pgtable_t check from vmemmap_pgtable_withdraw
> in here.
OK.
>
>
> > static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> > {
> > + /* Must be called before the initialization of @page->lru */
> > + vmemmap_pgtable_free(h, page);
> > +
> > INIT_LIST_HEAD(&page->lru);
> > set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > set_hugetlb_cgroup(page, NULL);
> > @@ -1783,6 +1892,14 @@ static struct page *alloc_fresh_huge_page(struct hstate *h,
> > if (!page)
> > return NULL;
> >
> > + if (vmemmap_pgtable_prealloc(h, page)) {
> > + if (hstate_is_gigantic(h))
> > + free_gigantic_page(page, huge_page_order(h));
> > + else
> > + put_page(page);
> > + return NULL;
> > + }
> > +
>
> I must confess I am bit puzzled.
>
> IIUC, in this patch we are just adding the helpers to create/tear the page
> tables.
> I do not think we actually need to call vmemmap_pgtable_prealloc/vmemmap_pgtable_free, do we?
> In the end, we are just allocating pages for pagetables and then free them shortly.
>
> I think it would make more sense to add the calls when they need to be?
OK, will do. Thanks very much.
>
>
> --
> Oscar Salvador
> SUSE L3
--
Yours,
Muchun
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