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

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

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On 11.03.2015 15:30, Konstantin Khlebnikov wrote:
> On Thu, Feb 26, 2015 at 4:51 PM, Vlastimil Babka <vbabka@suse.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@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;
>> +               }
>>          }
>>
>>          if (!page)
>> --
>> 2.1.4
>>
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shmem: show swap usage in smaps

From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 fs/proc/task_mmu.c |    3 +++
 include/linux/mm.h |    2 ++
 mm/shmem.c         |    8 ++++++++
 3 files changed, 13 insertions(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 956b75d61809..09a94cec159e 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -624,6 +624,9 @@ static int show_smap(struct seq_file *m, void *v, int is_pid)
 	/* mmap_sem is held in m_start */
 	walk_page_vma(vma, &smaps_walk);
 
+	if (vma->vm_ops && vma->vm_ops->get_swap_usage)
+		mss.swap += vma->vm_ops->get_swap_usage(vma) << PAGE_SHIFT;
+
 	show_map_vma(m, vma, is_pid);
 
 	seq_printf(m,
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6571dd78e984..477a46987859 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -292,6 +292,8 @@ struct vm_operations_struct {
 	 */
 	struct page *(*find_special_page)(struct vm_area_struct *vma,
 					  unsigned long addr);
+
+	unsigned long (*get_swap_usage)(struct vm_area_struct *vma);
 };
 
 struct mmu_gather;
diff --git a/mm/shmem.c b/mm/shmem.c
index cf2d0ca010bc..492f78f51fc2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1363,6 +1363,13 @@ static struct mempolicy *shmem_get_policy(struct vm_area_struct *vma,
 }
 #endif
 
+static unsigned long shmem_get_swap_usage(struct vm_area_struct *vma)
+{
+	struct inode *inode = file_inode(vma->vm_file);
+
+	return SHMEM_I(inode)->swapped;
+}
+
 int shmem_lock(struct file *file, int lock, struct user_struct *user)
 {
 	struct inode *inode = file_inode(file);
@@ -3198,6 +3205,7 @@ static const struct vm_operations_struct shmem_vm_ops = {
 	.set_policy     = shmem_set_policy,
 	.get_policy     = shmem_get_policy,
 #endif
+	.get_swap_usage	= shmem_get_swap_usage,
 };
 
 static struct dentry *shmem_mount(struct file_system_type *fs_type,