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

[Will Deacon]

On Fri, Sep 09, 2022 at 01:24:10AM +0300, Kirill A. Shutemov wrote:
> 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@xxxxxxxxx>
> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> 
> +Will.
> 
> Seems I confused update_mmu_tlb() with update_mmu_cache() :/

Urgh, that thing is pretty horrible! But anyway, I agree that this change
looks correct based on the other callers in the file.

> Looks good to me:
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

I'm assuming Andrew will pick this up. Otherwise, please let me know if
I should route it via the arm64 tree.

Will