Re: [PATCH v2 3/6] tlb: mmu_gather: Introduce tlb_gather_mmu_fullmm()

[Yu Zhao]

On Thu, Dec 10, 2020 at 12:11:07PM +0000, Will Deacon wrote:
> Passing the range '0, -1' to tlb_gather_mmu() sets the 'fullmm' flag,
> which indicates that the mm_struct being operated on is going away. In
> this case, some architectures (such as arm64) can elide TLB invalidation
> by ensuring that the TLB tag (ASID) associated with this mm is not
> immediately reclaimed. Although this behaviour is documented in
> asm-generic/tlb.h, it's subtle and easily missed.
> 
> Introduce tlb_gather_mmu_fullmm() to make it clearer that this is for the
> entire mm and WARN() if tlb_gather_mmu() is called with the 'fullmm'
> address range.
> 
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> ---
>  include/asm-generic/tlb.h |  6 ++++--
>  include/linux/mm_types.h  |  1 +
>  mm/mmap.c                 |  2 +-
>  mm/mmu_gather.c           | 16 ++++++++++++++--
>  4 files changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
> index 6661ee1cff47..2c68a545ffa7 100644
> --- a/include/asm-generic/tlb.h
> +++ b/include/asm-generic/tlb.h
> @@ -46,7 +46,9 @@
>   *
>   * The mmu_gather API consists of:
>   *
> - *  - tlb_gather_mmu() / tlb_finish_mmu(); start and finish a mmu_gather
> + *  - tlb_gather_mmu() / tlb_gather_mmu_fullmm() / tlb_finish_mmu()
> + *
> + *    start and finish a mmu_gather
>   *
>   *    Finish in particular will issue a (final) TLB invalidate and free
>   *    all (remaining) queued pages.
> @@ -91,7 +93,7 @@
>   *
>   *  - mmu_gather::fullmm
>   *
> - *    A flag set by tlb_gather_mmu() to indicate we're going to free
> + *    A flag set by tlb_gather_mmu_fullmm() to indicate we're going to free
>   *    the entire mm; this allows a number of optimizations.
>   *
>   *    - We can ignore tlb_{start,end}_vma(); because we don't
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 7b90058a62be..42231729affe 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -585,6 +585,7 @@ static inline cpumask_t *mm_cpumask(struct mm_struct *mm)
>  struct mmu_gather;
>  extern void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
>  				unsigned long start, unsigned long end);
> +extern void tlb_gather_mmu_fullmm(struct mmu_gather *tlb, struct mm_struct *mm);
>  extern void tlb_finish_mmu(struct mmu_gather *tlb);
>  
>  static inline void init_tlb_flush_pending(struct mm_struct *mm)
> diff --git a/mm/mmap.c b/mm/mmap.c
> index a3e5854cd01e..cdd3dae6547c 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3214,7 +3214,7 @@ void exit_mmap(struct mm_struct *mm)
>  
>  	lru_add_drain();
>  	flush_cache_mm(mm);
> -	tlb_gather_mmu(&tlb, mm, 0, -1);
> +	tlb_gather_mmu_fullmm(&tlb, mm);
>  	/* update_hiwater_rss(mm) here? but nobody should be looking */
>  	/* Use -1 here to ensure all VMAs in the mm are unmapped */
>  	unmap_vmas(&tlb, vma, 0, -1);
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index b0be5a7aa08f..5f5e45d9eb50 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -261,8 +261,8 @@ void tlb_flush_mmu(struct mmu_gather *tlb)
>   * respectively when @mm is without users and we're going to destroy
>   * the full address space (exit/execve).
>   */
> -void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
> -			unsigned long start, unsigned long end)
> +static void __tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
> +			     unsigned long start, unsigned long end)
>  {
>  	tlb->mm = mm;
>  
> @@ -287,6 +287,18 @@ void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
>  	inc_tlb_flush_pending(tlb->mm);
>  }
>  
> +void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
> +		    unsigned long start, unsigned long end)
> +{
> +	WARN_ON(!(start | (end + 1))); /* Use _fullmm() instead */
> +	__tlb_gather_mmu(tlb, mm, start, end);
> +}
> +
> +void tlb_gather_mmu_fullmm(struct mmu_gather *tlb, struct mm_struct *mm)
> +{
> +	__tlb_gather_mmu(tlb, mm, 0, -1);
> +}
> +

IMO, there is no point adding the wrappers given you will remove
the WARN_ON in the next patch.

But if you prefer, they should be at least moved to the header for
inlining.

Consider this whole series

Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx>