mm/filemap: fix mapping_seek_hole_data on THP & 32-bit

No problem on 64-bit, or 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).

[hughd@google.com: remove unneeded u64 casts, per Matthew]
  Link: https://lkml.kernel.org/r/alpine.LSU.2.11.2104221347240.1170@eggly.anvils

Link: https://lkml.kernel.org/r/alpine.LSU.2.11.2104211737410.3299@eggly.anvils
Fixes: 41139aa4 ("mm/filemap: add mapping_seek_hole_data")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -2678,7 +2678,7 @@ loff_t mapping_seek_hole_data(struct address_space *mapping, loff_t start,
 		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;
 
@@ -2687,7 +2687,8 @@ loff_t mapping_seek_hole_data(struct address_space *mapping, loff_t start,
 
 	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)
@@ -2695,25 +2696,25 @@ loff_t mapping_seek_hole_data(struct address_space *mapping, loff_t start,
 			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;