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Subject[PATCH 2/2] mm: adds NOSIGBUS extension for out-of-band shmem read
Date
Adds new flag MAP_NOSIGBUS of mmap() to specify the behavior of
"don't SIGBUS on read beyond i_size". This flag is only allowed
for read only shmem mapping.

If you use MAP_NOSIGBUS, and you access pages that don't have a backing
store, you will get zero pages, and they will NOT BE SYNCHRONIZED with
the backing store possibly later being updated.

Any user that uses MAP_NOSIGBUS had better just accept that it's not
compatible with expanding the shmem backing store later.

Signed-off-by: Ming Lin <mlin@kernel.org>
---
include/linux/mm.h | 2 ++
include/linux/mman.h | 1 +
include/uapi/asm-generic/mman-common.h | 1 +
mm/mmap.c | 3 +++
mm/shmem.c | 17 ++++++++++++++++-
5 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index e9d67bc..5d0e0dc 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -373,6 +373,8 @@ int __add_to_page_cache_locked(struct page *page, struct address_space *mapping,
# define VM_UFFD_MINOR VM_NONE
#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */

+#define VM_NOSIGBUS VM_FLAGS_BIT(38) /* Do not SIGBUS on out-of-band shmem read */
+
/* Bits set in the VMA until the stack is in its final location */
#define VM_STACK_INCOMPLETE_SETUP (VM_RAND_READ | VM_SEQ_READ)

diff --git a/include/linux/mman.h b/include/linux/mman.h
index b2cbae9..c966b08 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -154,6 +154,7 @@ static inline bool arch_validate_flags(unsigned long flags)
_calc_vm_trans(flags, MAP_DENYWRITE, VM_DENYWRITE ) |
_calc_vm_trans(flags, MAP_LOCKED, VM_LOCKED ) |
_calc_vm_trans(flags, MAP_SYNC, VM_SYNC ) |
+ _calc_vm_trans(flags, MAP_NOSIGBUS, VM_NOSIGBUS ) |
arch_calc_vm_flag_bits(flags);
}

diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
index f94f65d..55f4be0 100644
--- a/include/uapi/asm-generic/mman-common.h
+++ b/include/uapi/asm-generic/mman-common.h
@@ -29,6 +29,7 @@
#define MAP_HUGETLB 0x040000 /* create a huge page mapping */
#define MAP_SYNC 0x080000 /* perform synchronous page faults for the mapping */
#define MAP_FIXED_NOREPLACE 0x100000 /* MAP_FIXED which doesn't unmap underlying mapping */
+#define MAP_NOSIGBUS 0x200000 /* do not SIGBUS on out-of-band shmem read */

#define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be
* uninitialized */
diff --git a/mm/mmap.c b/mm/mmap.c
index 096bba4..69cd856 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1419,6 +1419,9 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
if (!len)
return -EINVAL;

+ if ((flags & MAP_NOSIGBUS) && ((prot & PROT_WRITE) || !shmem_file(file)))
+ return -EINVAL;
+
/*
* Does the application expect PROT_READ to imply PROT_EXEC?
*
diff --git a/mm/shmem.c b/mm/shmem.c
index 5d46611..5d15b08 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1812,7 +1812,22 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
repeat:
if (sgp <= SGP_CACHE &&
((loff_t)index << PAGE_SHIFT) >= i_size_read(inode)) {
- return -EINVAL;
+ if (!vma || !(vma->vm_flags & VM_NOSIGBUS))
+ return -EINVAL;
+
+ vma->vm_flags |= VM_MIXEDMAP;
+ /*
+ * Get zero page for MAP_NOSIGBUS mapping, which isn't
+ * coherent wrt shmem contents that are expanded and
+ * filled in later.
+ */
+ error = vm_insert_page(vma, (unsigned long)vmf->address,
+ ZERO_PAGE(0));
+ if (error)
+ return error;
+
+ *fault_type = VM_FAULT_NOPAGE;
+ return 0;
}

sbinfo = SHMEM_SB(inode->i_sb);
--
1.8.3.1
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