Re: [PATCH] powerpc: Fix mprotect on book3s32

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

++linux-mm to get some pointers on how to test such mmu_gather changes

Dave Vasilevsky via B4 Relay <devnull+dave.vasilevsky.ca@kernel.org>
writes:

> From: Dave Vasilevsky <dave@vasilevsky.ca>
>
> On 32-bit book3s with hash-MMUs, tlb_flush() was a no-op. This was
> unnoticed because all uses until recently were for unmaps, and thus
> handled by __tlb_remove_tlb_entry().
>
> After commit 4a18419f71cd ("mm/mprotect: use mmu_gather") in kernel 5.19,
> tlb_gather_mmu() started being used for mprotect as well. This caused
> mprotect to simply not work on these machines:
>
>   int *ptr = mmap(NULL, 4096, PROT_READ|PROT_WRITE,
>                   MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
>   *ptr = 1; // force HPTE to be created
>   mprotect(ptr, 4096, PROT_READ);
>   *ptr = 2; // should segfault, but succeeds

I am surprised how come this was not caught? Don't we have any straight
forward selftest for this?

Not just mprotect then right.. Many other MM paths must also be using
mmu_gather right?

>
> Fixed by making tlb_flush() actually flush TLB pages. This finally
> agrees with the behaviour of boot3s64's tlb_flush().
>
> Fixes: 4a18419f71cd ("mm/mprotect: use mmu_gather")
> Signed-off-by: Dave Vasilevsky <dave@vasilevsky.ca>
> ---
>  arch/powerpc/include/asm/book3s/32/tlbflush.h | 8 ++++++--
>  arch/powerpc/mm/book3s32/tlb.c                | 6 ++++++
>  2 files changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/32/tlbflush.h b/arch/powerpc/include/asm/book3s/32/tlbflush.h
> index e43534da5207aa3b0cb3c07b78e29b833c141f3f..b8c587ad2ea954f179246a57d6e86e45e91dcfdc 100644
> --- a/arch/powerpc/include/asm/book3s/32/tlbflush.h
> +++ b/arch/powerpc/include/asm/book3s/32/tlbflush.h
> @@ -11,6 +11,7 @@
>  void hash__flush_tlb_mm(struct mm_struct *mm);
>  void hash__flush_tlb_page(struct vm_area_struct *vma, unsigned long vmaddr);
>  void hash__flush_range(struct mm_struct *mm, unsigned long start, unsigned long end);
> +void hash__flush_gather(struct mmu_gather *tlb);
>  
>  #ifdef CONFIG_SMP
>  void _tlbie(unsigned long address);
> @@ -28,9 +29,12 @@ void _tlbia(void);
>   */
>  static inline void tlb_flush(struct mmu_gather *tlb)
>  {
> -	/* 603 needs to flush the whole TLB here since it doesn't use a hash table. */
> -	if (!mmu_has_feature(MMU_FTR_HPTE_TABLE))
> +	if (mmu_has_feature(MMU_FTR_HPTE_TABLE)) {
> +		hash__flush_gather(tlb);
> +	} else {
> +		/* 603 needs to flush the whole TLB here since it doesn't use a hash table. */
>  		_tlbia();
> +	}
>  }
>  
>  static inline void flush_range(struct mm_struct *mm, unsigned long start, unsigned long end)
> diff --git a/arch/powerpc/mm/book3s32/tlb.c b/arch/powerpc/mm/book3s32/tlb.c
> index 9ad6b56bfec96e989b96f027d075ad5812500854..3da95ecfbbb296303082e378425e92a5fbdbfac8 100644
> --- a/arch/powerpc/mm/book3s32/tlb.c
> +++ b/arch/powerpc/mm/book3s32/tlb.c
> @@ -105,3 +105,9 @@ void hash__flush_tlb_page(struct vm_area_struct *vma, unsigned long vmaddr)
>  		flush_hash_pages(mm->context.id, vmaddr, pmd_val(*pmd), 1);
>  }
>  EXPORT_SYMBOL(hash__flush_tlb_page);
> +
> +void hash__flush_gather(struct mmu_gather *tlb)
> +{
> +	hash__flush_range(tlb->mm, tlb->start, tlb->end);
> +}
> +EXPORT_SYMBOL(hash__flush_gather);

Shouldn't we flush all if we get tlb_flush request for full mm? e.g.
Something like this maybe? 

+void hash__tlb_flush(struct mmu_gather *tlb)
+{
+       if (tlb->fullmm || tlb->need_flush_all)
+               hash__flush_tlb_mm(tlb->mm);
+       else
+               hash__flush_range(tlb->mm, tlb->start, tlb->end);
+}

It will be quicker if someone already has a set of tests which we can
run to validate. If not, I will take a look and see what tests one can
run to validate mmu_gather feature. 

>
> ---
> base-commit: dcb6fa37fd7bc9c3d2b066329b0d27dedf8becaa
> change-id: 20251027-vasi-mprotect-g3-f8f5278d4140
>
> Best regards,
> -- 
> Dave Vasilevsky <dave@vasilevsky.ca>

Thanks again for pointing this out. How did you find this though?
What hardware do you use?

-ritesh