Re: [PATCH v3 2/4] mm, proc: account for shmem swap in /proc/pid/smaps
From: Michal Hocko <hidden>
Date: 2015-09-25 12:57:35
Also in:
linux-mm, lkml
[Sorry for a really long delay] On Wed 05-08-15 15:01:23, 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.
Yes I agree.
Signed-off-by: Vlastimil Babka <redacted>
[...]
quoted hunk ↗ jump to hunk
@@ -625,6 +626,41 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma) seq_putc(m, '\n'); } +#if defined(CONFIG_SHMEM) && defined(CONFIG_SWAP) +static unsigned long smaps_shmem_swap(struct vm_area_struct *vma) +{ + struct inode *inode; + unsigned long swapped; + pgoff_t start, end; + + if (!vma->vm_file) + return 0; + + inode = file_inode(vma->vm_file);
Why don't we need to take i_mutex here? What prevents from a parallel truncate? I guess we do not care because radix_tree_for_each_slot would cope with a truncated portion of the range, right? It would deserve a comment I guess.
+ + if (!shmem_mapping(inode->i_mapping)) + return 0; + + swapped = shmem_swap_usage(inode); + + if (swapped == 0) + return 0; + + if (vma->vm_end - vma->vm_start >= inode->i_size) + return swapped; + + start = linear_page_index(vma, vma->vm_start); + end = linear_page_index(vma, vma->vm_end); + + return shmem_partial_swap_usage(inode->i_mapping, start, end); +}
[...]
+unsigned long shmem_partial_swap_usage(struct address_space *mapping,
+ pgoff_t start, pgoff_t end)
+{
+ struct radix_tree_iter iter;
+ void **slot;
+ struct page *page;
+ unsigned long swapped = 0;
+
+ rcu_read_lock();
+
+restart:
+ radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
+ if (iter.index >= end)
+ break;
+
+ page = radix_tree_deref_slot(slot);
+
+ /*
+ * This should only be possible to happen at index 0, so we
+ * don't need to reset the counter, nor do we risk infinite
+ * restarts.
+ */
+ if (radix_tree_deref_retry(page))
+ goto restart;
+
+ if (radix_tree_exceptional_entry(page))
+ swapped++;
+
+ if (need_resched()) {
+ cond_resched_rcu();
+ start = iter.index + 1;
+ goto restart;
+ }
+ }
+
+ rcu_read_unlock();
+
+ return swapped << PAGE_SHIFT;
+}
+#endif
+
/*
* SysV IPC SHM_UNLOCK restore Unevictable pages to their evictable lists.
*/
--
2.4.6
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