On Tue 17-09-19 16:48:14, yangerkun wrote:
> No need to wait when offset equals to 0. And it will trigger a bug since
> the latter __ext4_journalled_invalidatepage can free the buffers but leave
> page still dirty.
>
> [ 26.057508] ------------[ cut here ]------------
> [ 26.058531] kernel BUG at fs/ext4/inode.c:2134!
> ...
> [ 26.088130] Call trace:
> [ 26.088695] ext4_writepage+0x914/0xb28
> [ 26.089541] writeout.isra.4+0x1b4/0x2b8
> [ 26.090409] move_to_new_page+0x3b0/0x568
> [ 26.091338] __unmap_and_move+0x648/0x988
> [ 26.092241] unmap_and_move+0x48c/0xbb8
> [ 26.093096] migrate_pages+0x220/0xb28
> [ 26.093945] kernel_mbind+0x828/0xa18
> [ 26.094791] __arm64_sys_mbind+0xc8/0x138
> [ 26.095716] el0_svc_common+0x190/0x490
> [ 26.096571] el0_svc_handler+0x60/0xd0
> [ 26.097423] el0_svc+0x8/0xc
>
> Run below parallel can reproduce it easily(ext3):
> void main()
> {
> int fd, fd1, fd2, fd3, ret;
> void *addr;
> size_t length = 4096;
> int flags;
> off_t offset = 0;
> char *str = "12345";
>
> fd = open("a", O_RDWR | O_CREAT);
> assert(fd >= 0);
>
> ret = ftruncate(fd, length);
> assert(ret == 0);
>
> fd1 = open("a", O_RDWR | O_CREAT, -1);
> assert(fd1 >= 0);
>
> flags = 0xc00f;/*Journal data mode*/
> ret = ioctl(fd1, _IOW('f', 2, long), &flags);
> assert(ret == 0);
>
> fd2 = open("a", O_RDWR | O_CREAT);
> assert(fd2 >= 0);
>
> fd3 = open("a", O_TRUNC | O_NOATIME);
> assert(fd3 >= 0);
>
> addr = mmap(NULL, length, 0xe, 0x28013, fd2, offset);
Ugh, these mmap flags look pretty bogus. Were they generated by some
fuzzer?
> assert(addr != (void *)-1);
> memcpy(addr, str, 5);
Also the O_TRUNC open above will truncate "a" to 0 so the mapping is
actually beyond i_size and this memcpy should fail with SIGBUS. So I'm
surprised your test program gets up to mbind()...
> mbind(addr, length, 0, 0, 0, 2);
>
> close(fd);
> munmap(addr, length);
> }
>
> Signed-off-by: yangerkun <yangerkun@xxxxxxxxxx>
I agree that there's no need to wait for transaction commit when offset ==
0. So your patch is correct in that regard. What still escapes me is why
this is necessary. I have a feeling that it just papers over the real
problem. You mention crash in ext4_writepage() because page is dirty but
has no buffers - but how come the page is dirty? If offset == 0 for a page,
truncate_inode_pages() should have cleaned PageDirty flag so the page
should never get to ext4_writepage() in the first place. Together with my
comments about the test case this is still a bit mystery to me... I guess
I'll try to reproduce this to understand this better.
Honza
> ---
> fs/ext4/inode.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 006b7a2070bf..a9943ae4f74d 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5479,7 +5479,7 @@ static void ext4_wait_for_tail_page_commit(struct inode *inode)
> * do. We do the check mainly to optimize the common PAGE_SIZE ==
> * blocksize case
> */
> - if (offset > PAGE_SIZE - i_blocksize(inode))
> + if (!offset || offset > PAGE_SIZE - i_blocksize(inode))
> return;
> while (1) {
> page = find_lock_page(inode->i_mapping,
> --
> 2.17.2
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
