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SubjectRe: [RFC][PATCH 1/3] pagemap: avoid splitting thp when reading /proc/pid/pagemap
On Mon, 19 Dec 2011, Naoya Horiguchi wrote:

> diff --git 3.2-rc5.orig/fs/proc/task_mmu.c 3.2-rc5/fs/proc/task_mmu.c
> index e418c5a..90c4b7a 100644
> --- 3.2-rc5.orig/fs/proc/task_mmu.c
> +++ 3.2-rc5/fs/proc/task_mmu.c
> @@ -600,6 +600,9 @@ struct pagemapread {
> u64 *buffer;
> };
>
> +#define PAGEMAP_WALK_SIZE (PMD_SIZE)
> +#define PAGEMAP_WALK_MASK (PMD_MASK)
> +
> #define PM_ENTRY_BYTES sizeof(u64)
> #define PM_STATUS_BITS 3
> #define PM_STATUS_OFFSET (64 - PM_STATUS_BITS)
> @@ -658,6 +661,22 @@ static u64 pte_to_pagemap_entry(pte_t pte)
> return pme;
> }
>
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static u64 thp_pte_to_pagemap_entry(pte_t pte, int offset)
> +{
> + u64 pme = 0;
> + if (pte_present(pte))
> + pme = PM_PFRAME(pte_pfn(pte) + offset)
> + | PM_PSHIFT(PAGE_SHIFT) | PM_PRESENT;
> + return pme;
> +}
> +#else
> +static inline u64 thp_pte_to_pagemap_entry(pte_t pte, int offset)
> +{
> + return 0;
> +}
> +#endif
> +
> static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> struct mm_walk *walk)
> {
> @@ -666,10 +685,33 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> pte_t *pte;
> int err = 0;
>
> - split_huge_page_pmd(walk->mm, pmd);
> -
> /* find the first VMA at or above 'addr' */
> vma = find_vma(walk->mm, addr);
> +
> + spin_lock(&walk->mm->page_table_lock);

pagemap_pte_range() could potentially be called a _lot_, so I'd recommend
optimizing this by checking for pmd_trans_huge() prior to taking
page_table_lock and then rechecking after grabbing it with likely().

> + if (pmd_trans_huge(*pmd)) {
> + if (pmd_trans_splitting(*pmd)) {
> + spin_unlock(&walk->mm->page_table_lock);
> + wait_split_huge_page(vma->anon_vma, pmd);
> + } else {
> + u64 pfn = PM_NOT_PRESENT;

This doesn't need to be initialized and it would probably be better to
declare "pfn" at the top-level of this function since it's later used for
the non-thp case.

> +
> + for (; addr != end; addr += PAGE_SIZE) {
> + int offset = (addr & ~PAGEMAP_WALK_MASK)
> + >> PAGE_SHIFT;
> + pfn = thp_pte_to_pagemap_entry(*(pte_t *)pmd,
> + offset);
> + err = add_to_pagemap(addr, pfn, pm);
> + if (err)
> + break;
> + }
> + spin_unlock(&walk->mm->page_table_lock);
> + return err;
> + }
> + } else {
> + spin_unlock(&walk->mm->page_table_lock);
> + }
> +
> for (; addr != end; addr += PAGE_SIZE) {
> u64 pfn = PM_NOT_PRESENT;
>
> @@ -754,8 +796,6 @@ static int pagemap_hugetlb_range(pte_t *pte, unsigned long hmask,
> * determine which areas of memory are actually mapped and llseek to
> * skip over unmapped regions.
> */
> -#define PAGEMAP_WALK_SIZE (PMD_SIZE)
> -#define PAGEMAP_WALK_MASK (PMD_MASK)
> static ssize_t pagemap_read(struct file *file, char __user *buf,
> size_t count, loff_t *ppos)
> {


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