Re: [PATCH 2/2] mm: adds NOSIGBUS extension for out-of-band shmem read

[Ming Lin <mlin@kernel.org>]

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?
  	 *