[PATCH] mm: pagemap: restrict pagewalk to the requested range

[Yuanchu Xie]

The pagewalk in pagemap_read reads one PTE past the end of the requested
range, and stops when the buffer runs out of space. While it produces
the right result, the extra read is unnecessary and less performant.

I timed the following command before and after this patch:
	dd count=100000 if=/proc/self/pagemap of=/dev/null
The results are consistently within 0.001s across 5 runs.

Before:
100000+0 records in
100000+0 records out
51200000 bytes (51 MB) copied, 0.0763159 s, 671 MB/s

real    0m0.078s
user    0m0.012s
sys     0m0.065s

After:
100000+0 records in
100000+0 records out
51200000 bytes (51 MB) copied, 0.0487928 s, 1.0 GB/s

real    0m0.050s
user    0m0.011s
sys     0m0.039s

Signed-off-by: Yuanchu Xie <yuanchu@xxxxxxxxxx>
---
 fs/proc/task_mmu.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 420510f6a545..6259dd432eeb 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1689,23 +1689,23 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
 	/* watch out for wraparound */
 	start_vaddr = end_vaddr;
 	if (svpfn <= (ULONG_MAX >> PAGE_SHIFT)) {
+		unsigned long end;
+
 		ret = mmap_read_lock_killable(mm);
 		if (ret)
 			goto out_free;
 		start_vaddr = untagged_addr_remote(mm, svpfn << PAGE_SHIFT);
 		mmap_read_unlock(mm);
+
+		end = start_vaddr + ((count / PM_ENTRY_BYTES) << PAGE_SHIFT);
+		if (end >= start_vaddr && end < mm->task_size)
+			end_vaddr = end;
 	}
 
 	/* Ensure the address is inside the task */
 	if (start_vaddr > mm->task_size)
 		start_vaddr = end_vaddr;
 
-	/*
-	 * The odds are that this will stop walking way
-	 * before end_vaddr, because the length of the
-	 * user buffer is tracked in "pm", and the walk
-	 * will stop when we hit the end of the buffer.
-	 */
 	ret = 0;
 	while (count && (start_vaddr < end_vaddr)) {
 		int len;
-- 
2.40.1.606.ga4b1b128d6-goog