Re: [PATCH v3 3/8] mm: drop per-VMA lock in handle_mm_fault if retrying or when finished

Peter Xu
2023-06-27T15:28:15Z

On Mon, Jun 26, 2023 at 09:23:16PM -0700, Suren Baghdasaryan wrote:
> handle_mm_fault returning VM_FAULT_RETRY or VM_FAULT_COMPLETED means
> mmap_lock has been released. However with per-VMA locks behavior is
> different and the caller should still release it. To make the
> rules consistent for the caller, drop the per-VMA lock before returning
> from handle_mm_fault when page fault should be retried or is completed.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> arch/arm64/mm/fault.c | 3 ++-
> arch/powerpc/mm/fault.c | 3 ++-
> arch/s390/mm/fault.c | 3 ++-
> arch/x86/mm/fault.c | 3 ++-
> mm/memory.c | 12 +++++++++++-
> 5 files changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 6045a5117ac1..89f84e9ea1ff 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -601,7 +601,8 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
> goto lock_mmap;
> }
> fault = handle_mm_fault(vma, addr, mm_flags | FAULT_FLAG_VMA_LOCK, regs);
> - vma_end_read(vma);
> + if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> + vma_end_read(vma);
>
> if (!(fault & VM_FAULT_RETRY)) {
> count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 531177a4ee08..4697c5dca31c 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -494,7 +494,8 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
> }
>
> fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
> - vma_end_read(vma);
> + if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> + vma_end_read(vma);
>
> if (!(fault & VM_FAULT_RETRY)) {
> count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index b65144c392b0..cccefe41038b 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -418,7 +418,8 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
> goto lock_mmap;
> }
> fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
> - vma_end_read(vma);
> + if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> + vma_end_read(vma);
> if (!(fault & VM_FAULT_RETRY)) {
> count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> goto out;
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index e4399983c50c..d69c85c1c04e 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1347,7 +1347,8 @@ void do_user_addr_fault(struct pt_regs *regs,
> goto lock_mmap;
> }
> fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
> - vma_end_read(vma);
> + if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> + vma_end_read(vma);
>
> if (!(fault & VM_FAULT_RETRY)) {
> count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> diff --git a/mm/memory.c b/mm/memory.c
> index f69fbc251198..9011ad63c41b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5086,7 +5086,17 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
> }
> }
>
> - return handle_pte_fault(&vmf);
> + ret = handle_pte_fault(&vmf);
> + if (ret & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)) {
> + /*
> + * In case of VM_FAULT_RETRY or VM_FAULT_COMPLETED we might
> + * be still holding per-VMA lock to keep the vma stable as long
> + * as possible. Drop it before returning.
> + */
> + if (vmf.flags & FAULT_FLAG_VMA_LOCK)
> + vma_end_read(vma);
> + }

This smells hackish.. I'd think better we just release the lock at the
place where we'll return RETRY, and AFAIU swap is the only place vma lock
returns a RETRY with current code base?

do_swap_page():
if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+ vma_end_read(vma);
ret = VM_FAULT_RETRY;
goto out;
}

I.e., I don't think VM_FAULT_COMPLETED can even be returned with vma lock
paths yet as it doesn't yet support VM_SHARED.

--
Peter Xu