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Subject[PATCH v2 1/4] mm: fix device-dax pud write-faults triggered by get_user_pages()
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Currently only get_user_pages_fast() can safely handle the writable gup
case due to its use of pud_access_permitted() to check whether the pud
entry is writable. In the gup slow path pud_write() is used instead of
pud_access_permitted() and to date it has been unimplemented, just calls
BUG_ON().

kernel BUG at ./include/linux/hugetlb.h:244!
[..]
RIP: 0010:follow_devmap_pud+0x482/0x490
[..]
Call Trace:
follow_page_mask+0x28c/0x6e0
__get_user_pages+0xe4/0x6c0
get_user_pages_unlocked+0x130/0x1b0
get_user_pages_fast+0x89/0xb0
iov_iter_get_pages_alloc+0x114/0x4a0
nfs_direct_read_schedule_iovec+0xd2/0x350
? nfs_start_io_direct+0x63/0x70
nfs_file_direct_read+0x1e0/0x250
nfs_file_read+0x90/0xc0

For now this just implements a simple check for the _PAGE_RW bit similar
to pmd_write. However, this implies that the gup-slow-path check is
missing the extra checks that the gup-fast-path performs with
pud_access_permitted. Later patches will align all checks to use the
'access_permitted' helper if the architecture provides it. Note that the
generic 'access_permitted' helper fallback is the simple _PAGE_RW check
on architectures that do not define the 'access_permitted' helper(s).

Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages")
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: <stable@vger.kernel.org>
Cc: <x86@kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
arch/arm64/include/asm/pgtable.h | 1 +
arch/sparc/include/asm/pgtable_64.h | 1 +
arch/x86/include/asm/pgtable.h | 6 ++++++
include/asm-generic/pgtable.h | 9 +++++++++
include/linux/hugetlb.h | 8 --------
5 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index b46e54c2399b..9a943792a823 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -340,6 +340,7 @@ static inline int pmd_protnone(pmd_t pmd)
#define pfn_pmd(pfn,prot) (__pmd(((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot)))
#define mk_pmd(page,prot) pfn_pmd(page_to_pfn(page),prot)

+#define __HAVE_ARCH_PUD_WRITE
#define pud_write(pud) pte_write(pud_pte(pud))
#define pud_pfn(pud) (((pud_val(pud) & PUD_MASK) & PHYS_MASK) >> PAGE_SHIFT)

diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
index fd9d9bac7cfa..bcf54a9cf6c5 100644
--- a/arch/sparc/include/asm/pgtable_64.h
+++ b/arch/sparc/include/asm/pgtable_64.h
@@ -693,6 +693,7 @@ static inline unsigned long pmd_write(pmd_t pmd)
return pte_write(pte);
}

+#define __HAVE_ARCH_PUD_WRITE
#define pud_write(pud) pte_write(__pte(pud_val(pud)))

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index f735c3016325..5c396724fd0d 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1093,6 +1093,12 @@ static inline void pmdp_set_wrprotect(struct mm_struct *mm,
clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp);
}

+#define __HAVE_ARCH_PUD_WRITE
+static inline int pud_write(pud_t pud)
+{
+ return pud_flags(pud) & _PAGE_RW;
+}
+
/*
* clone_pgd_range(pgd_t *dst, pgd_t *src, int count);
*
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index 757dc6ffc7ba..bd738624bd16 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -812,6 +812,15 @@ static inline int pmd_write(pmd_t pmd)
return 0;
}
#endif /* __HAVE_ARCH_PMD_WRITE */
+
+#ifndef __HAVE_ARCH_PUD_WRITE
+static inline int pud_write(pud_t pud)
+{
+ BUG();
+ return 0;
+}
+#endif /* __HAVE_ARCH_PUD_WRITE */
+
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

#if !defined(CONFIG_TRANSPARENT_HUGEPAGE) || \
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index fbf5b31d47ee..82a25880714a 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -239,14 +239,6 @@ static inline int pgd_write(pgd_t pgd)
}
#endif

-#ifndef pud_write
-static inline int pud_write(pud_t pud)
-{
- BUG();
- return 0;
-}
-#endif
-
#define HUGETLB_ANON_FILE "anon_hugepage"

enum {
\
 
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