Re: [PATCH 2/2] mm: adds NOSIGBUS extension for out-of-band shmem read
From: Ming Lin <mlin@kernel.org>
Date: 2021-06-03 19:57:06
Also in:
linux-fsdevel, linux-mm, lkml
Subsystem:
generic include/asm header files, memory management, memory management - core, memory mapping, memory mapping - madvise (memory advice), the rest · Maintainers:
Arnd Bergmann, Andrew Morton, David Hildenbrand, Liam R. Howlett, Lorenzo Stoakes, Linus Torvalds
On 6/2/2021 5:46 PM, Hugh Dickins wrote
It's do_anonymous_page()'s business to map in the zero page on read fault (see "my_zero_pfn(vmf->address)" in there), or fill a freshly allocated page with zeroes on write fault - and now you're sticking to MAP_PRIVATE, write faults in VM_WRITE areas are okay for VM_NOSIGBUS. Ideally you can simply call do_anonymous_page() from __do_fault() in the VM_FAULT_SIGBUS on VM_NOSIGBUS case. That's what to start from anyway: but look to see if there's state to be adjusted to achieve that; and it won't be surprising if somewhere down in do_anonymous_page() or something it calls, there's a BUG on it being called when vma->vm_file is set, or something like that. May need some tweaking.
do_anonymous_page() works nicely for read fault and write fault. I didn't see any BUG() thing in my test. But I'm still struggling with how to do "punch hole should remove the mapping of zero page". Here is the hack I have now.
diff --git a/mm/memory.c b/mm/memory.c
index 46ecda5..6b5a897 100644
--- a/mm/memory.c
+++ b/mm/memory.c@@ -1241,7 +1241,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, struct page *page; page = vm_normal_page(vma, addr, ptent); - if (unlikely(details) && page) { + if (unlikely(details) && page && !(vma->vm_flags & VM_NOSIGBUS)) { /* * unmap_shared_mapping_pages() wants to * invalidate cache without truncating:
And other parts of the patch is following, ----
diff --git a/include/linux/mm.h b/include/linux/mm.h
index e9d67bc..af9e277 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 fault */ + /* 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..a2a5333 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 fault */ #define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be * uninitialized */
diff --git a/mm/memory.c b/mm/memory.c
index eff2a47..46ecda5 100644
--- a/mm/memory.c
+++ b/mm/memory.c@@ -3676,6 +3676,17 @@ static vm_fault_t __do_fault(struct vm_fault *vmf) } ret = vma->vm_ops->fault(vmf); + if (unlikely(ret & VM_FAULT_SIGBUS) && (vma->vm_flags & VM_NOSIGBUS)) { + /* + * For MAP_NOSIGBUS mapping, map in the zero page on read fault + * or fill a freshly allocated page with zeroes on write fault + */ + ret = do_anonymous_page(vmf); + if (!ret) + ret = VM_FAULT_NOPAGE; + return ret; + } + if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | VM_FAULT_DONE_COW))) return ret;
diff --git a/mm/mmap.c b/mm/mmap.c
index 096bba4..74fb49a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c@@ -1419,6 +1419,10 @@ unsigned long do_mmap(struct file *file, unsigned long addr, if (!len) return -EINVAL; + /* Restrict MAP_NOSIGBUS to MAP_PRIVATE mapping */ + if ((flags & MAP_NOSIGBUS) && !(flags & MAP_PRIVATE)) + return -EINVAL; + /* * Does the application expect PROT_READ to imply PROT_EXEC? *