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Date: Wed, 20 Sep 2023 10:47:33 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Joao Martins <joao.m.martins@...cle.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>,
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 <21cnbao@...il.com>, Michal Hocko <mhocko@...e.com>,
Matthew Wilcox <willy@...radead.org>,
Xiongchun Duan <duanxiongchun@...edance.com>,
Linux-MM <linux-mm@...ck.org>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 6/8] hugetlb: batch PMD split for bulk vmemmap dedup
> On Sep 19, 2023, at 23:09, Joao Martins <joao.m.martins@...cle.com> wrote:
>
> On 19/09/2023 09:57, Muchun Song wrote:
>>> On Sep 19, 2023, at 16:55, Joao Martins <joao.m.martins@...cle.com> wrote:
>>> On 19/09/2023 09:41, Muchun Song wrote:
>>>>> On Sep 19, 2023, at 16:26, Joao Martins <joao.m.martins@...cle.com> wrote:
>>>>> On 19/09/2023 07:42, Muchun Song wrote:
>>>>>> On 2023/9/19 07:01, Mike Kravetz wrote:
>>>>>>> list_for_each_entry(folio, folio_list, lru) {
>>>>>>> int ret = __hugetlb_vmemmap_optimize(h, &folio->page,
>>>>>>> &vmemmap_pages);
>>>>>>
>>>>>> This is unlikely to be failed since the page table allocation
>>>>>> is moved to the above
>>>>>
>>>>>> (Note that the head vmemmap page allocation
>>>>>> is not mandatory).
>>>>>
>>>>> Good point that I almost forgot
>>>>>
>>>>>> So we should handle the error case in the above
>>>>>> splitting operation.
>>>>>
>>>>> But back to the previous discussion in v2... the thinking was that /some/ PMDs
>>>>> got split, and say could allow some PTE remapping to occur and free some pages
>>>>> back (each page allows 6 more splits worst case). Then the next
>>>>> __hugetlb_vmemmap_optimize() will have to split PMD pages again for those
>>>>> hugepages that failed the batch PMD split (as we only defer the PTE remap tlb
>>>>> flush in this stage).
>>>>
>>>> Oh, yes. Maybe we could break the above traversal as early as possible
>>>> once we enter an ENOMEM?
>>>>
>>>
>>> Sounds good -- no point in keep trying to split if we are failing with OOM.
>>>
>>> Perhaps a comment in both of these clauses (the early break on split and the OOM
>>> handling in batch optimize) could help make this clear.
>>
>> Make sense.
>
> These are the changes I have so far for this patch based on the discussion so
> far. For next one it's at the end:
Code looks good to me. One nit below.
>
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index e8bc2f7567db..d9c6f2cf698c 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -27,7 +27,8 @@
> * @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
> + * @flags: used to modify behavior in vmemmap page table walking
> + * operations.
> */
> struct vmemmap_remap_walk {
> void (*remap_pte)(pte_t *pte, unsigned long addr,
> @@ -36,6 +37,8 @@ struct vmemmap_remap_walk {
> struct page *reuse_page;
> unsigned long reuse_addr;
> struct list_head *vmemmap_pages;
> +
> +/* Skip the TLB flush when we split the PMD */
> #define VMEMMAP_SPLIT_NO_TLB_FLUSH BIT(0)
> unsigned long flags;
> };
> @@ -132,7 +135,7 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
> int ret;
>
> ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK,
> - walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH);
> + !(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH));
> if (ret)
> return ret;
>
> @@ -677,13 +680,13 @@ 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)
> +static int 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;
> + return 0;
>
> vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
> vmemmap_reuse = vmemmap_start;
> @@ -693,7 +696,7 @@ static void hugetlb_vmemmap_split(const struct hstate *h,
> struct page *head)
> * Split PMDs on the vmemmap virtual address range [@vmemmap_start,
> * @vmemmap_end]
> */
> - vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse);
> + return vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse);
> }
>
> void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head
> *folio_list)
> @@ -701,8 +704,18 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h,
> struct list_head *folio_l
> struct folio *folio;
> LIST_HEAD(vmemmap_pages);
>
> - list_for_each_entry(folio, folio_list, lru)
> - hugetlb_vmemmap_split(h, &folio->page);
> + list_for_each_entry(folio, folio_list, lru) {
> + int ret = hugetlb_vmemmap_split(h, &folio->page);
> +
> + /*
> + * Spliting the PMD requires allocating a page, thus lets fail
^^^^ ^^^
Splitting page table page
I'd like to specify the functionality of the allocated page.
> + * early once we encounter the first OOM. No point in retrying
> + * as it can be dynamically done on remap with the memory
> + * we get back from the vmemmap deduplication.
> + */
> + if (ret == -ENOMEM)
> + break;
> + }
>
> flush_tlb_all();
>
> For patch 7, I only have commentary added derived from this earlier discussion
> above:
>
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index d9c6f2cf698c..f6a1020a4b6a 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -40,6 +40,8 @@ struct vmemmap_remap_walk {
>
> /* Skip the TLB flush when we split the PMD */
> #define VMEMMAP_SPLIT_NO_TLB_FLUSH BIT(0)
> +/* Skip the TLB flush when we remap the PTE */
> #define VMEMMAP_REMAP_NO_TLB_FLUSH BIT(1)
> unsigned long flags;
> };
>
> @@ -721,19 +739,28 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h,
> struct list_head *folio_l
>
> list_for_each_entry(folio, folio_list, lru) {
> int ret = __hugetlb_vmemmap_optimize(h, &folio->page,
> &vmemmap_pages,
> VMEMMAP_REMAP_NO_TLB_FLUSH);
>
> /*
> * Pages to be freed may have been accumulated. If we
> * encounter an ENOMEM, free what we have and try again.
> + * This can occur in the case that both spliting fails
^^^
splitting
> + * halfway and head page allocation also failed. In this
^^^^^^^
head vmemmap page
Otherwise:
Reviewed-by: Muchun Song <songmuchun@...edance.com>
Thanks.
> + * case __hugetlb_vmemmap_optimize() would free memory
> + * allowing more vmemmap remaps to occur.
> */
> if (ret == -ENOMEM && !list_empty(&vmemmap_pages)) {
>
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