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