dax: disable pmd mappings
While dax pmd mappings are functional in the nominal path they trigger
kernel crashes in the following paths:
BUG: unable to handle kernel paging request at ffffea0004098000
IP: [<ffffffff812362f7>] follow_trans_huge_pmd+0x117/0x3b0
[..]
Call Trace:
[<ffffffff811f6573>] follow_page_mask+0x2d3/0x380
[<ffffffff811f6708>] __get_user_pages+0xe8/0x6f0
[<ffffffff811f7045>] get_user_pages_unlocked+0x165/0x1e0
[<ffffffff8106f5b1>] get_user_pages_fast+0xa1/0x1b0
kernel BUG at arch/x86/mm/gup.c:131!
[..]
Call Trace:
[<ffffffff8106f34c>] gup_pud_range+0x1bc/0x220
[<ffffffff8106f634>] get_user_pages_fast+0x124/0x1b0
BUG: unable to handle kernel paging request at ffffea0004088000
IP: [<ffffffff81235f49>] copy_huge_pmd+0x159/0x350
[..]
Call Trace:
[<ffffffff811fad3c>] copy_page_range+0x34c/0x9f0
[<ffffffff810a0daf>] copy_process+0x1b7f/0x1e10
[<ffffffff810a11c1>] _do_fork+0x91/0x590
All of these paths are interpreting a dax pmd mapping as a transparent
huge page and making the assumption that the pfn is covered by the
memmap, i.e. that the pfn has an associated struct page. PTE mappings
do not suffer the same fate since they have the _PAGE_SPECIAL flag to
cause the gup path to fault. We can do something similar for the PMD
path, or otherwise defer pmd support for cases where a struct page is
available. For now, 4.4-rc and -stable need to disable dax pmd support
by default.
For development the "depends on BROKEN" line can be removed from
CONFIG_FS_DAX_PMD.
Cc: <stable@vger.kernel.org>
Cc: Jan Kara <jack@suse.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Matthew Wilcox <willy@linux.intel.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
2 files changed