Re: [PATCH] mm: bring back update_mmu_cache() to finish_fault()

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Sep 08, 2022 at 11:48:09PM +0300, Sergei Antonov wrote:
> Running this test program on ARMv4 a few times (sometimes just once)
> reproduces the bug.
> 
> int main()
> {
>         unsigned i;
>         char paragon[SIZE];
>         void* ptr;
> 
>         memset(paragon, 0xAA, SIZE);
>         ptr = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
>                    MAP_ANON | MAP_SHARED, -1, 0);
>         if (ptr == MAP_FAILED) return 1;
>         printf("ptr = %p\n", ptr);
>         for (i=0;i<10000;i++){
>                 memset(ptr, 0xAA, SIZE);
>                 if (memcmp(ptr, paragon, SIZE)) {
>                         printf("Unexpected bytes on iteration %u!!!\n", i);
>                         break;
>                 }
>         }
>         munmap(ptr, SIZE);
> }
> 
> In the "ptr" buffer there appear runs of zero bytes which are aligned
> by 16 and their lengths are multiple of 16.
> 
> Linux v5.11 does not have the bug, "git bisect" finds the first bad commit:
> f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths")
> 
> Before the commit update_mmu_cache() was called during a call to
> filemap_map_pages() as well as finish_fault(). After the commit
> finish_fault() lacks it.
> 
> Bring back update_mmu_cache() to finish_fault() to fix the bug.
> Also call update_mmu_tlb() only when returning VM_FAULT_NOPAGE to more
> closely reproduce the code of alloc_set_pte() function that existed before
> the commit.
> 
> On many platforms update_mmu_cache() is nop:
>  x86, see arch/x86/include/asm/pgtable
>  ARMv6+, see arch/arm/include/asm/tlbflush.h
> So, it seems, few users ran into this bug.
> 
> Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths")
> Signed-off-by: Sergei Antonov <saproj@gmail.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

+Will.

Seems I confused update_mmu_tlb() with update_mmu_cache() :/

Looks good to me:

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

> ---
>  mm/memory.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 4ba73f5aa8bb..a78814413ac0 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4386,14 +4386,20 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>  
>  	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>  				      vmf->address, &vmf->ptl);
> -	ret = 0;
> +
>  	/* Re-check under ptl */
> -	if (likely(!vmf_pte_changed(vmf)))
> +	if (likely(!vmf_pte_changed(vmf))) {
>  		do_set_pte(vmf, page, vmf->address);
> -	else
> +
> +		/* no need to invalidate: a not-present page won't be cached */
> +		update_mmu_cache(vma, vmf->address, vmf->pte);
> +
> +		ret = 0;
> +	} else {
> +		update_mmu_tlb(vma, vmf->address, vmf->pte);
>  		ret = VM_FAULT_NOPAGE;
> +	}
>  
> -	update_mmu_tlb(vma, vmf->address, vmf->pte);
>  	pte_unmap_unlock(vmf->pte, vmf->ptl);
>  	return ret;
>  }
> -- 
> 2.34.1
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov