Re: [PATCH v2 2/2] mm: adds NOSIGBUS extension to mmap()
From: Vivek Goyal
Date: Mon Jun 28 2021 - 10:30:42 EST
On Fri, Jun 04, 2021 at 12:43:22AM -0700, Ming Lin wrote:
> Adds new flag MAP_NOSIGBUS of mmap() to specify the behavior of
> "don't SIGBUS on fault". Right now, this flag is only allowed
> for private mapping.
>
> For MAP_NOSIGBUS mapping, map in the zero page on read fault
> or fill a freshly allocated page with zeroes on write fault.
I am wondering if this could be of limited use for me if MAP_NOSIGBUS
were to be supported for shared mappings as well.
When virtiofs is run with dax enabled, then it is possible that if
a file is shared between two guests, then one guest truncates the
file and second guest tries to do load/store operation. Given current
kvm architecture, there is no mechanism to propagate SIGBUS to guest
process, instead KVM retries page fault infinitely and guest cpu/process
hangs.
Ideally we want this error to propagate all the way back into the
guest and to the guest process but that solution is not in place yet.
https://lore.kernel.org/kvm/20200406190951.GA19259@xxxxxxxxxx/
In the absense of a proper solution, one could think of mapping
shared file on host with MAP_NOSIGBUS, and hopefully that means
kvm will be able to resolve fault to a zero filled page and guest
will not hang. But this means that data sharing between two processes
is now broken. Writes by process A will not be visible to process B
in another once this situation happens, IIUC.
So if we were to MAP_NOSIGBUS, guest will not hang but failures resulting
from ftruncate will be silent and will be noticed sometime later. I guess
not exactly a very pleasant scenario...
Thanks
Vivek
>
> Signed-off-by: Ming Lin <mlin@xxxxxxxxxx>
> ---
> arch/parisc/include/uapi/asm/mman.h | 1 +
> include/linux/mm.h | 2 ++
> include/linux/mman.h | 1 +
> include/uapi/asm-generic/mman-common.h | 1 +
> mm/memory.c | 11 +++++++++++
> mm/mmap.c | 4 ++++
> tools/include/uapi/asm-generic/mman-common.h | 1 +
> 7 files changed, 21 insertions(+)
>
> diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h
> index ab78cba..eecf9af 100644
> --- a/arch/parisc/include/uapi/asm/mman.h
> +++ b/arch/parisc/include/uapi/asm/mman.h
> @@ -25,6 +25,7 @@
> #define MAP_STACK 0x40000 /* give out an address that is best suited for process/thread stacks */
> #define MAP_HUGETLB 0x80000 /* create a huge page 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 0 /* uninitialized anonymous mmap */
>
> #define MS_SYNC 1 /* synchronous memory sync */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9e86ca1..100d122 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 8d5e583..6b5a897 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 8bed547..d5c9fb5 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?
> *
> diff --git a/tools/include/uapi/asm-generic/mman-common.h b/tools/include/uapi/asm-generic/mman-common.h
> index f94f65d..a2a5333 100644
> --- a/tools/include/uapi/asm-generic/mman-common.h
> +++ b/tools/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 */
> --
> 1.8.3.1
>