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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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