On Fri, Apr 14, 2023 at 10:54:44AM -0700, Suren Baghdasaryan wrote:
> If the page fault handler requests a retry, we will count the fault
> multiple times. This is a relatively harmless problem as the retry paths
> are not often requested, and the only user-visible problem is that the
> fault counter will be slightly higher than it should be. Nevertheless,
> userspace only took one fault, and should not see the fact that the
> kernel had to retry the fault multiple times.
>
> Fixes: 6b4c9f446981 ("filemap: drop the mmap_sem for all blocking operations")
I know I suggested this fixes line, but I think it's actually been
here much longer, perhaps since
Fixes: d065bd810b6d ("mm: retry page fault when blocking on disk transfer")
Michel, what do you think?
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> Patch applies cleanly over linux-next and mm-unstable
>
> mm/memory.c | 16 ++++++++++------
> 1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 1c5b231fe6e3..d88f370eacd1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5212,17 +5212,16 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>
> __set_current_state(TASK_RUNNING);
>
> - count_vm_event(PGFAULT);
> - count_memcg_event_mm(vma->vm_mm, PGFAULT);
> -
> ret = sanitize_fault_flags(vma, &flags);
> if (ret)
> - return ret;
> + goto out;
>
> if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
> flags & FAULT_FLAG_INSTRUCTION,
> - flags & FAULT_FLAG_REMOTE))
> - return VM_FAULT_SIGSEGV;
> + flags & FAULT_FLAG_REMOTE)) {
> + ret = VM_FAULT_SIGSEGV;
> + goto out;
> + }
>
> /*
> * Enable the memcg OOM handling for faults triggered in user
> @@ -5253,6 +5252,11 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> }
>
> mm_account_fault(regs, address, flags, ret);
> +out:
> + if (!(ret & VM_FAULT_RETRY)) {
> + count_vm_event(PGFAULT);
> + count_memcg_event_mm(vma->vm_mm, PGFAULT);
> + }
>
> return ret;
> }
> --
> 2.40.0.634.g4ca3ef3211-goog
>
