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@xxxxxxxxxxxxxxx> Cc: Jan Kara <jack@xxxxxxxx> Cc: Dave Chinner <david@xxxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxxxx> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Reported-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> --- fs/Kconfig | 6 ++++++ fs/dax.c | 4 ++++ 2 files changed, 10 insertions(+) diff --git a/fs/Kconfig b/fs/Kconfig index da3f32f1a4e4..6ce72d8d1ee1 100644 --- a/fs/Kconfig +++ b/fs/Kconfig @@ -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 diff --git a/fs/dax.c b/fs/dax.c index d1e5cb7311a1..43671b68220e 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -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; -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
