Re: [PATCH -V3] /proc/PID/smaps: Add PMD migration entry parsing

[Michal Hocko]

On Fri 03-04-20 20:30:59, 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.
> 
> To test the patch, we run pmbench to eat 400 MB memory in background, then run
> /usr/bin/migratepages and `cat /proc/PID/smaps` every second.  The issue as
> follows can be reproduced within 60 seconds.
> 
> Before the patch, for the fully populated 400 MB anonymous VMA, some THP pages
> under migration may be lost as below.
> 
> 7f3f6a7e5000-7f3f837e5000 rw-p 00000000 00:00 0
> Size:             409600 kB
> KernelPageSize:        4 kB
> MMUPageSize:           4 kB
> Rss:              407552 kB
> Pss:              407552 kB
> Shared_Clean:          0 kB
> Shared_Dirty:          0 kB
> Private_Clean:         0 kB
> Private_Dirty:    407552 kB
> Referenced:       301056 kB
> Anonymous:        407552 kB
> LazyFree:              0 kB
> AnonHugePages:    405504 kB
> ShmemPmdMapped:        0 kB
> FilePmdMapped:        0 kB
> Shared_Hugetlb:        0 kB
> Private_Hugetlb:       0 kB
> Swap:                  0 kB
> SwapPss:               0 kB
> Locked:                0 kB
> THPeligible:		1
> VmFlags: rd wr mr mw me ac
> 
> After the patch, it will be always,
> 
> 7f3f6a7e5000-7f3f837e5000 rw-p 00000000 00:00 0
> Size:             409600 kB
> KernelPageSize:        4 kB
> MMUPageSize:           4 kB
> Rss:              409600 kB
> Pss:              409600 kB
> Shared_Clean:          0 kB
> Shared_Dirty:          0 kB
> Private_Clean:         0 kB
> Private_Dirty:    409600 kB
> Referenced:       294912 kB
> Anonymous:        409600 kB
> LazyFree:              0 kB
> AnonHugePages:    407552 kB
> ShmemPmdMapped:        0 kB
> FilePmdMapped:        0 kB
> Shared_Hugetlb:        0 kB
> Private_Hugetlb:       0 kB
> Swap:                  0 kB
> SwapPss:               0 kB
> Locked:                0 kB
> THPeligible:		1
> VmFlags: rd wr mr mw me ac
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
> 
> v3:
> 
> - Revised patch description and remove VM_WARN_ON_ONCE() per Michal's comments
> 
> v2:
> 
> - Use thp_migration_supported() in condition to reduce code size if THP
>   migration isn't enabled.
> 
> - Replace VM_BUG_ON() with VM_WARN_ON_ONCE(), it's not necessary to nuking
>   kernel for this.
> 
> ---
>  fs/proc/task_mmu.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 8d382d4ec067..36dc7417c0df 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -546,10 +546,17 @@ 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;
> +
> +	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);
>  
> -	/* FOLL_DUMP will return -EFAULT on huge zero page */
> -	page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> +		if (is_migration_entry(entry))
> +			page = migration_entry_to_page(entry);
> +	}
>  	if (IS_ERR_OR_NULL(page))
>  		return;
>  	if (PageAnon(page))
> @@ -578,8 +585,7 @@ 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