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

From: Hugh Dickins <hughd@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	William Kucharski <william.kucharski@oracle.com>,
	Christoph Hellwig <hch@lst.de>, Jan Kara <jack@suse.cz>,
	Dave Chinner <dchinner@redhat.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: [PATCH 2/2] mm/filemap: fix mapping_seek_hole_data on THP & 32-bit
Date: Wed, 21 Apr 2021 17:39:14 -0700 (PDT)	[thread overview]
Message-ID: <alpine.LSU.2.11.2104211737410.3299@eggly.anvils> (raw)
In-Reply-To: <alpine.LSU.2.11.2104211723580.3299@eggly.anvils>

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 casts added to stop unfortunate sign-extension when shifting
(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.  (But I believe
this use of round_up() assumes that any THP must be THP-aligned:
true while tmpfs enforces that alignment, and is the only fs with
FS_THP_SUPPORT; but might need to be generalized in the future?
If I try to generalize it right now, I'm sure to get it wrong!)

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>
---

 mm/filemap.c |   23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

--- 5.12-rc8/mm/filemap.c	2021-02-26 19:42:39.812156085 -0800
+++ linux/mm/filemap.c	2021-04-20 23:20:20.509464440 -0700
@@ -2671,8 +2671,8 @@ unsigned int seek_page_size(struct xa_st
 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;
+	XA_STATE(xas, &mapping->i_pages, (u64)start >> PAGE_SHIFT);
+	pgoff_t max = (u64)(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((u64)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, (u64)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;
