[PATCH v2 2/2] mm/filemap: fix mapping_seek_hole_data on THP & 32-bit

[Hugh Dickins <hughd@google.com>]

No problem on 64-bit without huge pages, but xfstests generic/285
and other SEEK_HOLE/SEEK_DATA tests have regressed on huge tmpfs,
and on 32-bit architectures, with the new mapping_seek_hole_data().
Several different bugs turned out to need fixing.

u64 cast to stop losing bits when converting unsigned long to loff_t
(and let's use shifts throughout, rather than mixed with * and /).

Use round_up() when advancing pos, to stop assuming that pos was
already THP-aligned when advancing it by THP-size.  (This use of
round_up() assumes that any THP has THP-aligned index: true at present
and true going forward, but could be recoded to avoid the assumption.)

Use xas_set() when iterating away from a THP, so that xa_index stays
in synch with start, instead of drifting away to return bogus offset.

Check start against end to avoid wrapping 32-bit xa_index to 0 (and
to handle these additional cases, seek_data or not, it's easier to
break the loop than goto: so rearrange exit from the function).

Fixes: 41139aa4c3a3 ("mm/filemap: add mapping_seek_hole_data")
Signed-off-by: Hugh Dickins <hughd@google.com>
---
v2: Removed all but one of v1's u64 casts, as suggested my Matthew.
    Updated commit message on u64 cast and THP alignment, per Matthew.

Andrew, I'd have just sent a -fix.patch to remove the unnecessary u64s,
but need to reword the commit message: so please replace yesterday's
mm-filemap-fix-mapping_seek_hole_data-on-thp-32-bit.patch
by this one - thanks.

 mm/filemap.c |   21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

--- 5.12-rc8/mm/filemap.c	2021-02-26 19:42:39.812156085 -0800
+++ linux/mm/filemap.c	2021-04-21 22:58:03.699655576 -0700
@@ -2672,7 +2672,7 @@ loff_t mapping_seek_hole_data(struct add
 		loff_t end, int whence)
 {
 	XA_STATE(xas, &mapping->i_pages, start >> PAGE_SHIFT);
-	pgoff_t max = (end - 1) / PAGE_SIZE;
+	pgoff_t max = (end - 1) >> PAGE_SHIFT;
 	bool seek_data = (whence == SEEK_DATA);
 	struct page *page;
 
@@ -2681,7 +2681,8 @@ loff_t mapping_seek_hole_data(struct add
 
 	rcu_read_lock();
 	while ((page = find_get_entry(&xas, max, XA_PRESENT))) {
-		loff_t pos = xas.xa_index * PAGE_SIZE;
+		loff_t pos = (u64)xas.xa_index << PAGE_SHIFT;
+		unsigned int seek_size;
 
 		if (start < pos) {
 			if (!seek_data)
@@ -2689,25 +2690,25 @@ loff_t mapping_seek_hole_data(struct add
 			start = pos;
 		}
 
-		pos += seek_page_size(&xas, page);
+		seek_size = seek_page_size(&xas, page);
+		pos = round_up(pos + 1, seek_size);
 		start = page_seek_hole_data(&xas, mapping, page, start, pos,
 				seek_data);
 		if (start < pos)
 			goto unlock;
+		if (start >= end)
+			break;
+		if (seek_size > PAGE_SIZE)
+			xas_set(&xas, pos >> PAGE_SHIFT);
 		if (!xa_is_value(page))
 			put_page(page);
 	}
-	rcu_read_unlock();
-
 	if (seek_data)
-		return -ENXIO;
-	goto out;
-
+		start = -ENXIO;
 unlock:
 	rcu_read_unlock();
-	if (!xa_is_value(page))
+	if (page && !xa_is_value(page))
 		put_page(page);
-out:
 	if (start > end)
 		return end;
 	return start;