On Thu 02-04-20 10:00:31, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
>
> Now, when read /proc/PID/smaps, the PMD migration entry in page table is simply
> ignored. To improve the accuracy of /proc/PID/smaps, its parsing and processing
> is added.
>
> Before the patch, for a fully populated 400 MB anonymous VMA, sometimes some THP
> pages under migration may be lost as follows.
Interesting. How did you reproduce this?
[...]
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 8d382d4ec067..9c72f9ce2dd8 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -546,10 +546,19 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
> struct mem_size_stats *mss = walk->private;
> struct vm_area_struct *vma = walk->vma;
> bool locked = !!(vma->vm_flags & VM_LOCKED);
> - struct page *page;
> + struct page *page = NULL;
>
> - /* FOLL_DUMP will return -EFAULT on huge zero page */
> - page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> + if (pmd_present(*pmd)) {
> + /* FOLL_DUMP will return -EFAULT on huge zero page */
> + page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> + } else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
> + swp_entry_t entry = pmd_to_swp_entry(*pmd);
> +
> + if (is_migration_entry(entry))
> + page = migration_entry_to_page(entry);
> + else
> + VM_WARN_ON_ONCE(1);
Could you explain why do we need this WARN_ON? I haven't really checked
the swap support for THP but cannot we have normal swap pmd entries?
> + }
> if (IS_ERR_OR_NULL(page))
> return;
> if (PageAnon(page))
> @@ -578,8 +587,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>
> ptl = pmd_trans_huge_lock(pmd, vma);
> if (ptl) {
> - if (pmd_present(*pmd))
> - smaps_pmd_entry(pmd, addr, walk);
> + smaps_pmd_entry(pmd, addr, walk);
> spin_unlock(ptl);
> goto out;
> }
> --
> 2.25.0
--
Michal Hocko
SUSE Labs
