Re: [PATCH 2/8] dax: disable pmd mappings

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Tue, Nov 17, 2015 at 12:16:03PM -0800, Dan Williams wrote:
> 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>

Acked-by: Ross Zwisler <ross.zwisler@linux.intel.com>