Re: [PATCH v3 5/8] mm: make folio_lock_fault indicate the state of mmap_lock upon return

[Alistair Popple <apopple@nvidia.com>]

Suren Baghdasaryan <surenb@google.com> writes:

> folio_lock_fault might drop mmap_lock before returning and to extend it
> to work with per-VMA locks, the callers will need to know whether the
> lock was dropped or is still held. Introduce new fault_flag to indicate
> whether the lock got dropped and store it inside vm_fault flags.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  include/linux/mm_types.h | 1 +
>  mm/filemap.c             | 2 ++
>  2 files changed, 3 insertions(+)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 79765e3dd8f3..6f0dbef7aa1f 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1169,6 +1169,7 @@ enum fault_flag {
>  	FAULT_FLAG_UNSHARE =		1 << 10,
>  	FAULT_FLAG_ORIG_PTE_VALID =	1 << 11,
>  	FAULT_FLAG_VMA_LOCK =		1 << 12,
> +	FAULT_FLAG_LOCK_DROPPED =	1 << 13,

Minor nit but this should also be added to the enum documentation
comment above this.

>  };
>  
>  typedef unsigned int __bitwise zap_flags_t;
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 87b335a93530..8ad06d69895b 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1723,6 +1723,7 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
>  			return VM_FAULT_RETRY;
>  
>  		mmap_read_unlock(mm);
> +		vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
>  		if (vmf->flags & FAULT_FLAG_KILLABLE)
>  			folio_wait_locked_killable(folio);
>  		else
> @@ -1735,6 +1736,7 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
>  		ret = __folio_lock_killable(folio);
>  		if (ret) {
>  			mmap_read_unlock(mm);
> +			vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
>  			return VM_FAULT_RETRY;
>  		}
>  	} else {