[PATCH] After swapout/swapin private dirty mappings become clean

[Nikanth Karthikesan <knikanth@suse.de>]

/proc/$pid/smaps broken: After swapout/swapin private dirty mappings become
clean.

When a page with private file mapping becomes dirty, the vma will be in both
i_mmap tree and anon_vma list. The /proc/$pid/smaps will account these pages
as dirty and backed by the file.

But when those dirty pages gets swapped out, and when they are read back from
swap, they would be marked as clean, as it should be, as they are part of swap
cache now.

But the /proc/$pid/smaps would report the vma as a mapping of a file and it is
clean. The pages are actually in same state i.e., dirty with respect to file
still, but which was once reported as dirty is now being reported as clean to
user-space.

This confuses tools like gdb which uses this information. Those tools think
that those pages were never modified and it creates problem when they create
dumps.

The file mapping of the vma also cannot be broken as pages never read earlier,
will still have to come from the file. Just that those dirty pages have become
clean anonymous pages.

During swaping in, restoring the exact state as dirty file-backed pages before
swapout would be useless, as there in no real bug. Breaking the vma with only
anonymous pages as seperate vmas unnecessary may not be a good thing as well.
So let us just export the information that a file-backed vma has anonymous
dirty pages.

Export this information in smaps by prepending file-names with "[anon]+", when
some of the pages in a file backed vma become anonymous.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>

---

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 439fc1f..68f9806 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -242,6 +242,8 @@ static void show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
 	 */
 	if (file) {
 		pad_len_spaces(m, len);
+		if (vma->anon_vma)
+			seq_puts(m, "[anon]+");
 		seq_path(m, &file->f_path, "\n");
 	} else {
 		const char *name = arch_vma_name(vma);