mm: pagemap: restrict pagewalk to the requested range

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

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yuanchu Xie <[email protected]>
Acked-by: Peter Xu <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Zach O'Keefe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: yuanchu-xie

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;