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Message-ID: <e20692b8-ae64-b2e9-4177-062bf0c937ba@oracle.com>
Date: Wed, 30 Aug 2023 12:13:33 +0100
From: Joao Martins <joao.m.martins@...cle.com>
To: Muchun Song <muchun.song@...ux.dev>,
Mike Kravetz <mike.kravetz@...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 30/08/2023 09:09, Muchun Song wrote:
>
>
> 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)
bulk here has a meaning of PMD being split in bulk, not the bulk of the vmemmap
pages. But yeah it's weird, I should clean this up.
> 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.
>
Based on the feedback of another patch I think it's abetter as you say, to
introduce a VMEMMAP_NO_TLB_FLUSH, and use the the walk::remap_pte to tell
whether it's a PMD split or a PTE remap.
>> } 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.
>
Yeap, great suggestion.
>> +
>> 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);
>
Yeap.
>> 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?
>
Yeap, a leftover from something else.
>> + 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.
>
OK.
>> + 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.
>
Yes, you're right. It's wrong, it was a copy-and-paste error from
remap_free and I failed to remove the non vmemmap_optimize specific
logic.
> 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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