Re: [PATCH 2/4] mm, procfs: account for shmem swap in /proc/pid/smaps

[Jerome Marchand <jmarchan@redhat.com>]

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On 02/26/2015 02:51 PM, Vlastimil Babka wrote:
> Currently, /proc/pid/smaps will always show "Swap: 0 kB" for shmem-backed
> mappings, even if the mapped portion does contain pages that were swapped out.
> This is because unlike private anonymous mappings, shmem does not change pte
> to swap entry, but pte_none when swapping the page out. In the smaps page
> walk, such page thus looks like it was never faulted in.
> 
> This patch changes smaps_pte_entry() to determine the swap status for such
> pte_none entries for shmem mappings, similarly to how mincore_page() does it.
> Swapped out pages are thus accounted for.
> 
> The accounting is arguably still not as precise as for private anonymous
> mappings, since now we will count also pages that the process in question never
> accessed, but only another process populated them and then let them become
> swapped out. I believe it is still less confusing and subtle than not showing
> any swap usage by shmem mappings at all. Also, swapped out pages only becomee a
> performance issue for future accesses, and we cannot predict those for neither
> kind of mapping.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  Documentation/filesystems/proc.txt |  3 ++-
>  fs/proc/task_mmu.c                 | 20 ++++++++++++++++++++
>  2 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index d4f56ec..8b30543 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -437,7 +437,8 @@ indicates the amount of memory currently marked as referenced or accessed.
>  a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
>  and a page is modified, the file page is replaced by a private anonymous copy.
>  "Swap" shows how much would-be-anonymous memory is also used, but out on
> -swap.
> +swap. For shmem mappings, "Swap" shows how much of the mapped portion of the
> +underlying shmem object is on swap.
>  
>  "VmFlags" field deserves a separate description. This member represents the kernel
>  flags associated with the particular virtual memory area in two letter encoded
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 956b75d..0410309 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -13,6 +13,7 @@
>  #include <linux/swap.h>
>  #include <linux/swapops.h>
>  #include <linux/mmu_notifier.h>
> +#include <linux/shmem_fs.h>
>  
>  #include <asm/elf.h>
>  #include <asm/uaccess.h>
> @@ -496,6 +497,25 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>  			mss->swap += PAGE_SIZE;
>  		else if (is_migration_entry(swpent))
>  			page = migration_entry_to_page(swpent);
> +	} else if (IS_ENABLED(CONFIG_SHMEM) && IS_ENABLED(CONFIG_SWAP) &&
> +					pte_none(*pte) && vma->vm_file) {
> +		struct address_space *mapping =
> +			file_inode(vma->vm_file)->i_mapping;
> +
> +		/*
> +		 * shmem does not use swap pte's so we have to consult
> +		 * the radix tree to account for swap
> +		 */
> +		if (shmem_mapping(mapping)) {
> +			page = find_get_entry(mapping, pgoff);
> +			if (page) {
> +				if (radix_tree_exceptional_entry(page))
> +					mss->swap += PAGE_SIZE;
> +				else
> +					page_cache_release(page);
> +			}
> +			page = NULL;
> +		}

Hi Vlastimil,

I'm afraid that isn't enough. Without walking the pte holes too, big
chunks of swapped out shmem pages may be missed.

Jerome

>  	}
>  
>  	if (!page)
> 



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