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Date: Wed, 30 Aug 2023 16:09:58 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Mike Kravetz <mike.kravetz@...cle.com>,
Joao Martins <joao.m.martins@...cle.com>
Cc: Muchun Song <songmuchun@...edance.com>,
Oscar Salvador <osalvador@...e.de>,
David Hildenbrand <david@...hat.com>,
Miaohe Lin <linmiaohe@...wei.com>,
David Rientjes <rientjes@...gle.com>,
Anshuman Khandual <anshuman.khandual@....com>,
Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
Barry Song <song.bao.hua@...ilicon.com>,
Michal Hocko <mhocko@...e.com>,
Matthew Wilcox <willy@...radead.org>,
Xiongchun Duan <duanxiongchun@...edance.com>,
Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/12] hugetlb: batch PMD split for bulk vmemmap dedup
On 2023/8/26 03:04, Mike Kravetz wrote:
> From: Joao Martins <joao.m.martins@...cle.com>
>
> In an effort to minimize amount of TLB flushes, batch all PMD splits
> belonging to a range of pages in order to perform only 1 (global) TLB
> flush. This brings down from 14.2secs into 7.9secs a 1T hugetlb
> allocation.
>
> Rebased by Mike Kravetz
>
> Signed-off-by: Joao Martins <joao.m.martins@...cle.com>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
> mm/hugetlb_vmemmap.c | 94 ++++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 90 insertions(+), 4 deletions(-)
>
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 500a118915ff..904a64fe5669 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -26,6 +26,7 @@
> * @reuse_addr: the virtual address of the @reuse_page page.
> * @vmemmap_pages: the list head of the vmemmap pages that can be freed
> * or is mapped from.
> + * @flags used to modify behavior in bulk operations
> */
> struct vmemmap_remap_walk {
> void (*remap_pte)(pte_t *pte, unsigned long addr,
> @@ -34,9 +35,11 @@ struct vmemmap_remap_walk {
> struct page *reuse_page;
> unsigned long reuse_addr;
> struct list_head *vmemmap_pages;
> +#define VMEMMAP_REMAP_ONLY_SPLIT BIT(0)
> + unsigned long flags;
> };
>
> -static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
> +static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start, bool bulk)
> {
> pmd_t __pmd;
> int i;
> @@ -79,7 +82,8 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
> /* Make pte visible before pmd. See comment in pmd_install(). */
> smp_wmb();
> pmd_populate_kernel(&init_mm, pmd, pgtable);
> - flush_tlb_kernel_range(start, start + PMD_SIZE);
> + if (!bulk)
> + flush_tlb_kernel_range(start, start + PMD_SIZE);
A little weird to me. @bulk is used to indicate whether the TLB
should be flushed, however, the flag name is VMEMMAP_REMAP_ONLY_SPLIT,
it seems to tell me @bulk (calculated from walk->flags &
VMEMMAP_REMAP_ONLY_SPLIT)
is a indicator to only split the huge pmd entry. For me, I think
it is better to introduce another flag like VMEMMAP_SPLIT_WITHOUT_FLUSH
to indicate whether TLB should be flushed.
> } else {
> pte_free_kernel(&init_mm, pgtable);
> }
> @@ -119,18 +123,28 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
> unsigned long end,
> struct vmemmap_remap_walk *walk)
> {
> + bool bulk;
> pmd_t *pmd;
> unsigned long next;
>
> + bulk = walk->flags & VMEMMAP_REMAP_ONLY_SPLIT;
> pmd = pmd_offset(pud, addr);
> do {
> int ret;
>
> - ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK);
> + ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK, bulk);
> if (ret)
> return ret;
>
> next = pmd_addr_end(addr, end);
> +
> + /*
> + * We are only splitting, not remapping the hugetlb vmemmap
> + * pages.
> + */
> + if (bulk)
> + continue;
Actually, we don not need a flag to detect this situation, you could
use "!@...k->remap_pte" to determine whether we should go into the
next level traversal of the page table. ->remap_pte is used to traverse
the pte entry, so it make senses to continue to the next pmd entry if
it is NULL.
> +
> vmemmap_pte_range(pmd, addr, next, walk);
> } while (pmd++, addr = next, addr != end);
>
> @@ -197,7 +211,8 @@ static int vmemmap_remap_range(unsigned long start, unsigned long end,
> return ret;
> } while (pgd++, addr = next, addr != end);
>
> - flush_tlb_kernel_range(start, end);
> + if (!(walk->flags & VMEMMAP_REMAP_ONLY_SPLIT))
> + flush_tlb_kernel_range(start, end);
This could be:
if (walk->remap_pte)
flush_tlb_kernel_range(start, end);
>
> return 0;
> }
> @@ -296,6 +311,48 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> }
>
> +/**
> + * vmemmap_remap_split - split the vmemmap virtual address range [@start, @end)
> + * backing PMDs of the directmap into PTEs
> + * @start: start address of the vmemmap virtual address range that we want
> + * to remap.
> + * @end: end address of the vmemmap virtual address range that we want to
> + * remap.
> + * @reuse: reuse address.
> + *
> + * Return: %0 on success, negative error code otherwise.
> + */
> +static int vmemmap_remap_split(unsigned long start, unsigned long end,
> + unsigned long reuse)
> +{
> + int ret;
> + LIST_HEAD(vmemmap_pages);
Unused variable?
> + struct vmemmap_remap_walk walk = {
> + .flags = VMEMMAP_REMAP_ONLY_SPLIT,
> + };
> +
> + /*
> + * In order to make remapping routine most efficient for the huge pages,
> + * the routine of vmemmap page table walking has the following rules
> + * (see more details from the vmemmap_pte_range()):
> + *
> + * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
> + * should be continuous.
> + * - The @reuse address is part of the range [@reuse, @end) that we are
> + * walking which is passed to vmemmap_remap_range().
> + * - The @reuse address is the first in the complete range.
> + *
> + * So we need to make sure that @start and @reuse meet the above rules.
> + */
The comments are duplicated, something like:
/* See the comment in the vmemmap_remap_free(). */
is enough.
> + BUG_ON(start - reuse != PAGE_SIZE);
> +
> + mmap_read_lock(&init_mm);
> + ret = vmemmap_remap_range(reuse, end, &walk);
> + mmap_read_unlock(&init_mm);
> +
> + return ret;
> +}
> +
> /**
> * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
> * to the page which @reuse is mapped to, then free vmemmap
> @@ -320,6 +377,7 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
> .remap_pte = vmemmap_remap_pte,
> .reuse_addr = reuse,
> .vmemmap_pages = &vmemmap_pages,
> + .flags = 0,
> };
> int nid = page_to_nid((struct page *)start);
> gfp_t gfp_mask = GFP_KERNEL | __GFP_THISNODE | __GFP_NORETRY |
> @@ -606,11 +664,39 @@ void hugetlb_vmemmap_optimize_bulk(const struct hstate *h, struct page *head,
> __hugetlb_vmemmap_optimize(h, head, bulk_pages);
> }
>
> +void hugetlb_vmemmap_split(const struct hstate *h, struct page *head)
> +{
> + unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
> + unsigned long vmemmap_reuse;
> +
> + if (!vmemmap_should_optimize(h, head))
> + return;
> +
> + static_branch_inc(&hugetlb_optimize_vmemmap_key);
Why? hugetlb_optimize_vmemmap_key is used as a switch to let
page_fixed_fake_head works properly for the vmemmap-optimized
HugeTLB pages, however, this function only splits the huge pmd
entry without optimizing the vmemmap pages. So it is wrong to
increase the static_key.
Thanks.
> +
> + vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
> + vmemmap_reuse = vmemmap_start;
> + vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE;
> +
> + /*
> + * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end)
> + * to the page which @vmemmap_reuse is mapped to, then free the pages
> + * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
> + */
> + if (vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse))
> + static_branch_dec(&hugetlb_optimize_vmemmap_key);
> +}
> +
> void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_list)
> {
> struct folio *folio;
> LIST_HEAD(vmemmap_pages);
>
> + list_for_each_entry(folio, folio_list, lru)
> + hugetlb_vmemmap_split(h, &folio->page);
> +
> + flush_tlb_kernel_range(0, TLB_FLUSH_ALL);
> +
> list_for_each_entry(folio, folio_list, lru)
> hugetlb_vmemmap_optimize_bulk(h, &folio->page, &vmemmap_pages);
>
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