[PATCH v5 2/6] mm, proc: account for shmem swap in /proc/pid/smaps

[Vlastimil Babka <vbabka@suse.cz>]

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 shmem pages are thus accounted for. For private mappings of tmpfs
files that COWed some of the pages, swaped out status of the original shmem
pages is naturally ignored. If some of the private copies was also swapped
out, they are accounted via their page table swap entries, so the resulting
reported swap usage is then a sum of both swapped out private copies, and
swapped out shmem pages that were not COWed. No double accounting can thus
happen.

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 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. Swapped out counter might of interest of users
who would like to prevent from future swapins during performance critical
operation and pre-fault them at their convenience. Especially for larger
swapped out regions the cost of swapin is much higher than a fresh page
allocation.  So a differentiation between pte_none vs. swapped out is important
for those usecases.

One downside of this patch is that it makes /proc/pid/smaps more expensive for
shmem mappings, as we consult the radix tree for each pte_none entry, so the
overal complexity is O(n*log(n)). I have measured this on a process that
creates a 2GB mapping and dirties single pages with a stride of 2MB, and time
how long does it take to cat /proc/pid/smaps of this process 100 times.

Private anonymous mapping:

real    0m0.949s
user    0m0.116s
sys     0m0.348s

Mapping of a /dev/shm/file:

real    0m3.831s
user    0m0.180s
sys     0m3.212s

The difference rather substantional, so the next patch will reduce the cost
for shared or read-only mappings.

In a less controlled experiment, I've gathered pids of processes on my desktop
that have either '/dev/shm/*' or 'SYSV*' in smaps. This included the Chrome
browser and some KDE processes. Again, I've run cat /proc/pid/smaps on each
100 times.

Before this patch:

real    0m9.050s
user    0m0.518s
sys     0m8.066s

After this patch:

real    0m9.221s
user    0m0.541s
sys     0m8.187s

This suggests low impact on average systems.

Note that this patch doesn't attempt to adjust the SwapPss field for shmem
mappings, which would need extra work to determine who else could have the
pages mapped. Thus the value stays zero except for COWed swapped out pages in
a shmem mapping, which are accounted as usual.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 Documentation/filesystems/proc.txt |  5 +++-
 fs/proc/task_mmu.c                 | 51 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 9f13b6e..fdeb5b3 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -460,7 +460,10 @@ and a page is modified, the file page is replaced by a private anonymous copy.
 hugetlbfs page which is *not* counted in "RSS" or "PSS" field for historical
 reasons. And these are not included in {Shared,Private}_{Clean,Dirty} field.
 "Swap" shows how much would-be-anonymous memory is also used, but out on swap.
-"SwapPss" shows proportional swap share of this mapping.
+For shmem mappings, "Swap" includes also the size of the mapped (and not
+replaced by copy-on-write) part of the underlying shmem object out on swap.
+"SwapPss" shows proportional swap share of this mapping. Unlike "Swap", this
+does not take into account swapped out page of underlying shmem objects.
 "Locked" indicates whether the mapping is locked in memory or not.
 
 "VmFlags" field deserves a separate description. This member represents the kernel
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 9e0938b..7e0c4c2 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -451,6 +451,7 @@ struct mem_size_stats {
 	unsigned long private_hugetlb;
 	u64 pss;
 	u64 swap_pss;
+	bool check_shmem_swap;
 };
 
 static void smaps_account(struct mem_size_stats *mss, struct page *page,
@@ -500,6 +501,45 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
 	}
 }
 
+#ifdef CONFIG_SHMEM
+static unsigned long smaps_shmem_swap(struct vm_area_struct *vma,
+		unsigned long addr)
+{
+	struct page *page;
+
+	page = find_get_entry(vma->vm_file->f_mapping,
+					linear_page_index(vma, addr));
+	if (!page)
+		return 0;
+
+	if (radix_tree_exceptional_entry(page))
+		return PAGE_SIZE;
+
+	page_cache_release(page);
+	return 0;
+
+}
+
+static int smaps_pte_hole(unsigned long addr, unsigned long end,
+		struct mm_walk *walk)
+{
+	struct mem_size_stats *mss = walk->private;
+
+	while (addr < end) {
+		mss->swap += smaps_shmem_swap(walk->vma, addr);
+		addr += PAGE_SIZE;
+	}
+
+	return 0;
+}
+#else
+static unsigned long smaps_shmem_swap(struct vm_area_struct *vma,
+		unsigned long addr)
+{
+	return 0;
+}
+#endif
+
 static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 		struct mm_walk *walk)
 {
@@ -527,6 +567,9 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 			}
 		} else if (is_migration_entry(swpent))
 			page = migration_entry_to_page(swpent);
+	} else if (unlikely(IS_ENABLED(CONFIG_SHMEM) && mss->check_shmem_swap
+							&& pte_none(*pte))) {
+		mss->swap += smaps_shmem_swap(vma, addr);
 	}
 
 	if (!page)
@@ -686,6 +729,14 @@ static int show_smap(struct seq_file *m, void *v, int is_pid)
 	};
 
 	memset(&mss, 0, sizeof mss);
+
+#ifdef CONFIG_SHMEM
+	if (vma->vm_file && shmem_mapping(vma->vm_file->f_mapping)) {
+		mss.check_shmem_swap = true;
+		smaps_walk.pte_hole = smaps_pte_hole;
+	}
+#endif
+
 	/* mmap_sem is held in m_start */
 	walk_page_vma(vma, &smaps_walk);
 
-- 
2.6.3

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