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dax: disable pmd mappings

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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: <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Reported-by: Ross Zwisler <[email protected]>
Signed-off-by: Dan Williams <[email protected]>
This commit is contained in:
Dan Williams 2015-11-15 16:06:32 -08:00
commit ee82c9ed41
2 changed files with 10 additions and 0 deletions
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6
fs/Kconfig
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@ -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