proc/kcore: optimize multiple page reads

The current code does a full search of the segment list every time for
every page.  This is wasteful, since it's almost certain that the next
page will be in the same segment.  Instead, check if the previous segment
covers the current page before doing the list search.

Link: http://lkml.kernel.org/r/fd346c11090cf93d867e01b8d73a6567c5ac6361.1531953780.git.osandov@fb.com
Signed-off-by: Omar Sandoval <osandov@fb.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Bhupesh Sharma <bhsharma@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: James Morse <james.morse@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index 808ef9afd084004928f64327151b8fcae02aa9a5..758c14e46a44ebbd1121e6080a1b53157ef8b3c2 100644 (file)
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -428,10 +428,18 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
        if ((tsz = (PAGE_SIZE - (start & ~PAGE_MASK))) > buflen)
                tsz = buflen;
 
+       m = NULL;
        while (buflen) {
-               list_for_each_entry(m, &kclist_head, list) {
-                       if (start >= m->addr && start < (m->addr+m->size))
-                               break;
+               /*
+                * If this is the first iteration or the address is not within
+                * the previous entry, search for a matching entry.
+                */
+               if (!m || start < m->addr || start >= m->addr + m->size) {
+                       list_for_each_entry(m, &kclist_head, list) {
+                               if (start >= m->addr &&
+                                   start < m->addr + m->size)
+                                       break;
+                       }
                }
 
                if (&m->list == &kclist_head) {