linux-kernel - Re: [PATCH] mm: bring back update_mmu_cache() to finish

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Date:   Fri, 9 Sep 2022 07:34:34 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Sergei Antonov <saproj@...il.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm: bring back update_mmu_cache() to finish_fault()

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@...il.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  mm/memory.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>