diff mbox series

Fix zero_user_segments() with start > end

Message ID 87eeh2erm0.fsf@mail.parknet.co.jp (mailing list archive)
State New
Headers show
Series Fix zero_user_segments() with start > end | expand

Commit Message

OGAWA Hirofumi Feb. 26, 2021, 4:11 p.m. UTC
zero_user_segments() is used from __block_write_begin_int(), for
example like the following

	zero_user_segments(page, 4096, 1024, 512, 918)

But new zero_user_segments() implements for HIGMEM + TRANSPARENT_HUGEPAGE 
doesn't handle "start > end" case correctly, and hits BUG_ON(). (we
can fix __block_write_begin_int() instead though, it is the old and
multiple usage)

Also it calls kmap_atomic() unnecessary while start == end == 0.

Cc: <stable@vger.kernel.org>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
---
 mm/highmem.c |   17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

Comments

Matthew Wilcox Feb. 26, 2021, 8:51 p.m. UTC | #1
On Sat, Feb 27, 2021 at 01:11:35AM +0900, OGAWA Hirofumi wrote:
> zero_user_segments() is used from __block_write_begin_int(), for
> example like the following
> 
> 	zero_user_segments(page, 4096, 1024, 512, 918)
> 
> But new zero_user_segments() implements for HIGMEM + TRANSPARENT_HUGEPAGE 
> doesn't handle "start > end" case correctly, and hits BUG_ON(). (we
> can fix __block_write_begin_int() instead though, it is the old and
> multiple usage)

Why don't we just take out the BUG_ON instead?  The function doesn't
actually do the wrong thing.

> Also it calls kmap_atomic() unnecessary while start == end == 0.

I'm OK with that.  It always used to do that.

> Cc: <stable@vger.kernel.org>
> Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

Fixes: 0060ef3b4e6d ("mm: support THPs in zero_user_segments")
OGAWA Hirofumi Feb. 27, 2021, 3:36 a.m. UTC | #2
Matthew Wilcox <willy@infradead.org> writes:

> On Sat, Feb 27, 2021 at 01:11:35AM +0900, OGAWA Hirofumi wrote:
>> zero_user_segments() is used from __block_write_begin_int(), for
>> example like the following
>> 
>> 	zero_user_segments(page, 4096, 1024, 512, 918)
>> 
>> But new zero_user_segments() implements for HIGMEM + TRANSPARENT_HUGEPAGE 
>> doesn't handle "start > end" case correctly, and hits BUG_ON(). (we
>> can fix __block_write_begin_int() instead though, it is the old and
>> multiple usage)
>
> Why don't we just take out the BUG_ON instead?  The function doesn't
> actually do the wrong thing.

end1 is underflow with

		if (start1 >= PAGE_SIZE) {
			start1 -= PAGE_SIZE;
			end1 -= PAGE_SIZE;
		}

>> Also it calls kmap_atomic() unnecessary while start == end == 0.
>
> I'm OK with that.  It always used to do that.

Old one is only one page, so it is always necessary if start1/end1 or
start2/end2 is valid range. But this one is multiple pages, so there are
completely unnecessary pages possibly.

>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
>
> Fixes: 0060ef3b4e6d ("mm: support THPs in zero_user_segments")

OK.
diff mbox series

Patch

diff --git a/mm/highmem.c b/mm/highmem.c
index 874b732..86f2b94 100644
--- a/mm/highmem.c	2021-02-20 12:56:49.037165666 +0900
+++ b/mm/highmem.c	2021-02-20 22:03:08.369361223 +0900
@@ -368,20 +368,24 @@  void zero_user_segments(struct page *pag
 
 	BUG_ON(end1 > page_size(page) || end2 > page_size(page));
 
+	if (start1 >= end1)
+		start1 = end1 = 0;
+	if (start2 >= end2)
+		start2 = end2 = 0;
+
 	for (i = 0; i < compound_nr(page); i++) {
 		void *kaddr = NULL;
 
-		if (start1 < PAGE_SIZE || start2 < PAGE_SIZE)
-			kaddr = kmap_atomic(page + i);
-
 		if (start1 >= PAGE_SIZE) {
 			start1 -= PAGE_SIZE;
 			end1 -= PAGE_SIZE;
 		} else {
 			unsigned this_end = min_t(unsigned, end1, PAGE_SIZE);
 
-			if (end1 > start1)
+			if (end1 > start1) {
+				kaddr = kmap_atomic(page + i);
 				memset(kaddr + start1, 0, this_end - start1);
+			}
 			end1 -= this_end;
 			start1 = 0;
 		}
@@ -392,8 +396,11 @@  void zero_user_segments(struct page *pag
 		} else {
 			unsigned this_end = min_t(unsigned, end2, PAGE_SIZE);
 
-			if (end2 > start2)
+			if (end2 > start2) {
+				if (!kaddr)
+					kaddr = kmap_atomic(page + i);
 				memset(kaddr + start2, 0, this_end - start2);
+			}
 			end2 -= this_end;
 			start2 = 0;
 		}