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Date: Wed, 6 Sep 2023 17:32:15 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Joao Martins <joao.m.martins@...cle.com>
Cc: Muchun Song <songmuchun@...edance.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>,
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>,
Michal Hocko <mhocko@...e.com>,
Matthew Wilcox <willy@...radead.org>,
Xiongchun Duan <duanxiongchun@...edance.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [External] [PATCH v2 09/11] hugetlb: batch PMD split for bulk
vmemmap dedup
> On Sep 6, 2023, at 17:26, Joao Martins <joao.m.martins@...cle.com> wrote:
>
>
>
> On 06/09/2023 10:11, Muchun Song wrote:
>> On Wed, Sep 6, 2023 at 4:25 PM Muchun Song <muchun.song@...ux.dev> wrote:
>>>
>>>
>>>
>>> On 2023/9/6 05:44, 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.
>>>>
>>>> Rebased and updated 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 | 72 +++++++++++++++++++++++++++++++++++++++++---
>>>> 1 file changed, 68 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>>>> index a715712df831..d956551699bc 100644
>>>> --- a/mm/hugetlb_vmemmap.c
>>>> +++ b/mm/hugetlb_vmemmap.c
>>>> @@ -37,7 +37,7 @@ struct vmemmap_remap_walk {
>>>> struct list_head *vmemmap_pages;
>>>> };
>>>>
>>>> -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 flush)
>>>> {
>>>> pmd_t __pmd;
>>>> int i;
>>>> @@ -80,7 +80,8 @@ 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 (flush)
>>>> + flush_tlb_kernel_range(start, start + PMD_SIZE);
>>>> } else {
>>>> pte_free_kernel(&init_mm, pgtable);
>>>> }
>>>> @@ -127,11 +128,20 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
>>>> do {
>>>> int ret;
>>>>
>>>> - ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK);
>>>> + ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK,
>>>> + walk->remap_pte != NULL);
>>>
>>> It is bettter to only make @walk->remap_pte indicate whether we should go
>>> to the last page table level. I suggest reusing VMEMMAP_NO_TLB_FLUSH
>>> to indicate whether we should flush the TLB at pmd level. It'll be more
>>> clear.
>>>
>>>> if (ret)
>>>> return ret;
>>>>
>>>> next = pmd_addr_end(addr, end);
>>>> +
>>>> + /*
>>>> + * We are only splitting, not remapping the hugetlb vmemmap
>>>> + * pages.
>>>> + */
>>>> + if (!walk->remap_pte)
>>>> + continue;
>>>> +
>>>> vmemmap_pte_range(pmd, addr, next, walk);
>>>> } while (pmd++, addr = next, addr != end);
>>>>
>>>> @@ -198,7 +208,8 @@ 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->remap_pte)
>>>> + flush_tlb_kernel_range(start, end);
>>>>
>>>> return 0;
>>>> }
>>>> @@ -297,6 +308,35 @@ 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;
>>>> + struct vmemmap_remap_walk walk = {
>>>> + .remap_pte = NULL,
>>>> + };
>>>> +
>>>> + /* See the comment in the vmemmap_remap_free(). */
>>>> + 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
>>>> @@ -602,11 +642,35 @@ void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
>>>> free_vmemmap_page_list(&vmemmap_pages);
>>>> }
>>>>
>>>> +static 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;
>>>> +
>>>> + vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
>>>> + vmemmap_reuse = vmemmap_start;
>>>> + vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE;
>>>> +
>>>> + /*
>>>> + * Split PMDs on the vmemmap virtual address range [@vmemmap_start,
>>>> + * @vmemmap_end]
>>>> + */
>>>> + vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse);
>>>> +}
>>>> +
>>>> 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);
>>>
>>> Maybe it is reasonable to add a return value to hugetlb_vmemmap_split()
>>> to indicate whether it has done successfully, if it fails, it must be
>>> OOM, in which case, there is no sense to continue to split the page table
>>> and optimize the vmemmap pages subsequently, right?
>>
>> Sorry, it is reasonable to continue to optimize the vmemmap pages
>> subsequently since it should succeed because those vmemmap pages
>> have been split successfully previously.
>>
>> Seems we should continue to optimize vmemmap once hugetlb_vmemmap_split()
>> fails, then we will have more memory to continue to split.
>
> Good point
>
>> But it will
>> make hugetlb_vmemmap_optimize_folios() a little complex. I'd like to
>> hear you guys' opinions here.
>>
> I think it won't add that much complexity if we don't optimize too much of the
> slowpath (when we are out of memory). In the batch freeing patch we could
> additionally test the return value of __hugetlb_vmemmap_optimize() for ENOMEM
> and free the currently stored vmemmap_pages (if any), and keep iterating the
> optimize loop. Should be simple enough and make this a bit more resilient to
> that scenario.
Yep, we could try this.
> But we would need to keep the earlier check you commented above
> (where we use @remap_pte to defer PMD flush).
I think 2 flags will suitable for you, one is VMEMMAP_REMAP_NO_TLB_FLUSH,
another is VMEMMAP_SPLIT_NO_TLB_FLUSH.
Thanks.
>
>> Thanks.
>>
>>>
>>> Thanks.
>>>
>>>> +
>>>> + flush_tlb_all();
>>>> +
>>>> list_for_each_entry(folio, folio_list, lru)
>>>> __hugetlb_vmemmap_optimize(h, &folio->page, &vmemmap_pages);
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