Message-ID: <5500592D.4090309@yandex-team.ru>
Date:	Wed, 11 Mar 2015 18:03:09 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Konstantin Khlebnikov <koct9i@...il.com>,
	Vlastimil Babka <vbabka@...e.cz>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
	Michal Hocko <mhocko@...e.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	linux-s390@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 2/4] mm, procfs: account for shmem swap in /proc/pid/smaps

On 11.03.2015 15:30, Konstantin Khlebnikov wrote:
> On Thu, Feb 26, 2015 at 4:51 PM, Vlastimil Babka <vbabka@...e.cz> 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.
>
> Maybe just add count of swap entries allocated by mapped shmem into
> swap usage of this vma? That's isn't exactly correct for partially
> mapped shmem but this is something weird anyway.

Something like that (see patch in attachment)

>
>>
>> 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@...e.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;
>> +               }
>>          }
>>
>>          if (!page)
>> --
>> 2.1.4
>>
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