Re: [PATCH 3/5] mm: sparsemem: split the huge PMD mapping of vmemmap pages

[Mike Kravetz]

On 6/9/21 5:13 AM, Muchun Song wrote:
> If the vmemmap is huge PMD mapped, we should split the huge PMD firstly
> and then we can change the PTE page table entry. In this patch, we add
> the ability of splitting the huge PMD mapping of vmemmap pages.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  include/linux/mm.h   |  2 +-
>  mm/hugetlb.c         | 42 ++++++++++++++++++++++++++++++++++--
>  mm/hugetlb_vmemmap.c |  3 ++-
>  mm/sparse-vmemmap.c  | 61 +++++++++++++++++++++++++++++++++++++++++++++-------
>  4 files changed, 96 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index cadc8cc2c715..b97e1486c5c1 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3056,7 +3056,7 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
>  #endif
>  
>  void vmemmap_remap_free(unsigned long start, unsigned long end,
> -			unsigned long reuse);
> +			unsigned long reuse, struct list_head *pgtables);
>  int vmemmap_remap_alloc(unsigned long start, unsigned long end,
>  			unsigned long reuse, gfp_t gfp_mask);
>  
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index c3b2a8a494d6..3137c72d9cc7 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1609,6 +1609,13 @@ static void __prep_account_new_huge_page(struct hstate *h, int nid)
>  static void __prep_new_huge_page(struct hstate *h, struct page *page)
>  {
>  	free_huge_page_vmemmap(h, page);
> +	/*
> +	 * Because we store preallocated pages on @page->lru,
> +	 * vmemmap_pgtable_free() must be called before the
> +	 * initialization of @page->lru in INIT_LIST_HEAD().
> +	 */
> +	vmemmap_pgtable_free(&page->lru);
> +
>  	INIT_LIST_HEAD(&page->lru);
>  	set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
>  	hugetlb_set_page_subpool(page, NULL);
> @@ -1775,14 +1782,29 @@ static struct page *alloc_fresh_huge_page(struct hstate *h,
>  		nodemask_t *node_alloc_noretry)
>  {
>  	struct page *page;
> +	LIST_HEAD(pgtables);
> +
> +	if (vmemmap_pgtable_prealloc(h, &pgtables))
> +		return NULL;

In the previous two patches I asked:
- Can we wait until later to prealloc vmemmap pages for gigantic pages
  allocated from bootmem?
- Should we fail to add a hugetlb page to the pool if we can not do
  vmemmap optimization?


Depending on the answers to those questions, we may be able to eliminate
these vmemmap_pgtable_prealloc/vmemmap_pgtable_free calls in hugetlb.c.
What about adding the calls to free_huge_page_vmemmap?
At the beginning of free_huge_page_vmemmap, allocate any vmemmap pgtable
pages.  If it fails, skip optimization.  We can free any pages before
returning to the caller.

Since we also know the page/address in the page table can we check to see
if it is already PTE mapped.  If so, can we then skip allocation?
-- 
Mike Kravetz