Re: [PATCH v4] Add /proc/PID/smaps support for DAX

[Dan Williams]

On Wed, Oct 25, 2017 at 10:13 PM, Fan Du <fan.du@xxxxxxxxx> wrote:
>
> Memory behind device DAX is not attached into normal memory
> management system, when user mmap /dev/dax, smaps part is
> currently missing, so no idea for user to check how much
> device DAX memory are actually used in practice.
>
> Whether vma is backed up by normal page,huge page, or both
> at the same time, this makes no difference for device DAX
> user so far.
>
> Using existing smaps structure is enough to do the job, so
> this patch tries to use existing RSS/PSS stuff for statistics.
> An example reading is like this:
> ----------------------------------------------------
> 7f30fe200000-7f3102200000 rw-s 00000000 00:06 19567                      /dev/dax12.0
> Size:              65536 kB
> KernelPageSize:        4 kB
> MMUPageSize:           4 kB
> Rss:               65536 kB
> Pss:               65536 kB
> Shared_Clean:          0 kB
> Shared_Dirty:          0 kB
> Private_Clean:         0 kB
> Private_Dirty:     65536 kB
> Referenced:        65536 kB
> Anonymous:             0 kB
> LazyFree:              0 kB
> AnonHugePages:         0 kB
> ShmemPmdMapped:        0 kB
> Shared_Hugetlb:        0 kB
> Private_Hugetlb:       0 kB
> Swap:                  0 kB
> SwapPss:               0 kB
> Locked:            65536 kB
> ProtectionKey:         0
> VmFlags: rd wr sh mr mw me ms mm hg
>
> Signed-off-by: Fan Du <fan.du@xxxxxxxxx>
> ---
> v4:
> * Merge device DAX readings into existing smap counters
>   for simplicity.
>
> v3:
> * Elaborate more about the usage suggested by Michal Hocko
>
> v2:
> * Using pte_devmap to check valid pfn page structure,
>   Pointed out by Dan. thx!
>  fs/proc/task_mmu.c | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 72 insertions(+), 2 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 5589b4b..9b2d3e6 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -507,6 +507,55 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
>         }
>  }
>
> +/* page structure behind DAX mappings is NOT compound page
> + * when it's a huge page mappings, so introduce new API to
> + * account for both PMD and PUD mapping.
> + */
> +static void smaps_account_dax_huge(struct mem_size_stats *mss,
> +                       struct page *page, unsigned long size, bool young, bool dirty)
> +{
> +       int mapcount = page_mapcount(page);
> +
> +       if (PageAnon(page)) {
> +               mss->anonymous += size;
> +               if (!PageSwapBacked(page) && !dirty && !PageDirty(page))
> +                       mss->lazyfree += size;
> +       }
> +
> +       mss->resident += size;
> +       /* Accumulate the size in pages that have been accessed. */
> +       if (young || page_is_young(page) || PageReferenced(page))
> +               mss->referenced += size;
> +
> +       /*
> +        * page_count(page) == 1 guarantees the page is mapped exactly once.
> +        * If any subpage of the compound page mapped with PTE it would elevate
> +        * page_count().
> +        */
> +       if (page_count(page) == 1) {
> +               if (dirty || PageDirty(page))
> +                       mss->private_dirty += size;
> +               else
> +                       mss->private_clean += size;
> +               mss->pss += (u64)size << PSS_SHIFT;
> +               return;
> +       }
> +
> +       if (mapcount >= 2) {
> +               if (dirty || PageDirty(page))
> +                       mss->shared_dirty += size;
> +               else
> +                       mss->shared_clean += size;
> +               mss->pss += (size << PSS_SHIFT) / mapcount;
> +       } else {
> +               if (dirty || PageDirty(page))
> +                       mss->private_dirty += size;
> +               else
> +                       mss->private_clean += size;
> +               mss->pss += size << PSS_SHIFT;
> +       }
> +}
> +
>  #ifdef CONFIG_SHMEM
>  static int smaps_pte_hole(unsigned long addr, unsigned long end,
>                 struct mm_walk *walk)
> @@ -528,7 +577,16 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>         struct page *page = NULL;
>
>         if (pte_present(*pte)) {
> -               page = vm_normal_page(vma, addr, *pte);
> +               if (!vma_is_dax(vma))
> +                       page = vm_normal_page(vma, addr, *pte);
> +               else if (pte_devmap(*pte)) {
> +                       struct dev_pagemap *pgmap;
> +
> +                       pgmap = get_dev_pagemap(pte_pfn(*pte), NULL);

Where do you do the put_dev_pagemap?