Re: [PATCH] Fix zero_user_segments() with start > end

[OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>]

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.
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>