Commit ee82c9ed authored Nov 15, 2015 by Dan Williams

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>

parent ef83b6e8

fs/Kconfig

+6 −0

Original line number	Diff line number	Diff line
		@@ -46,6 +46,12 @@ config FS_DAX
		or if unsure, say N. Saying Y will increase the size of the kernel
		by about 5kB.

		config FS_DAX_PMD
		bool
		default FS_DAX
		depends on FS_DAX
		depends on BROKEN

		endif # BLOCK

		# Posix ACL utility routines

fs/dax.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -541,6 +541,10 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
		unsigned long pfn;
		int result = 0;

		/* dax pmd mappings are broken wrt gup and fork */
		if (!IS_ENABLED(CONFIG_FS_DAX_PMD))
		return VM_FAULT_FALLBACK;

		/* Fall back to PTEs if we're going to COW */
		if (write && !(vma->vm_flags & VM_SHARED))
		return VM_FAULT_FALLBACK;

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