[Bug 124651] ext4 bugon panic when I mmap a file

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=124651

--- Comment #2 from wangguang <35445233@qq.com> ---
fs/ext4/inode.c:2263：mpage_prepare_extent_to_map：

          2261：        lblk = ((ext4_lblk_t)page->index) <<
                (PAGE_CACHE_SHIFT - blkbits);
          2263：        head = page_buffers(page);



EXT4 BUG_ON page_buffers() because a page goes to writeback without
buffer-heads attached.

I think unmap_page_range set the page dirty again Cause the problem。

I can recurrent the bug by:
（1） mmap a file 
 addr = (char *)mmap(NULL, MAPLEN, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
 memset(addr, 'i', MAPLEN);
（2） return err at
ext4_writepages->mpage_map_and_submit_extent->mpage_map_one_extent  Cause the
log:
        ext4_msg(sb, KERN_CRIT,
                 "Delayed block allocation failed for "
                 "inode %lu at logical offset %llu with"
                 " max blocks %u with error %d",
                 inode->i_ino,
                 (unsigned long long)map->m_lblk,
                 (unsigned)map->m_len, -err);

(3） Unmap case warning:
WARNING: at fs/buffer.c:661 __set_page_dirty+0xaa/0xc0()

(4) wait for sync，then bugon

see the attachment please.

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