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

[Sergei Antonov <saproj@xxxxxxxxx>]

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>
---
 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