> On Dec 24, 2018, at 12:49 AM, Michal Hocko <mhocko@xxxxxxxx> wrote: > > [Cc-ing mailing list and people involved in the original patch] > > On Fri 21-12-18 13:42:24, Paul Oppenheimer wrote: >> Hello! I've never reported a kernel bug before, and since its on the >> "next" tree I was told to email the author of the relevant commit. >> Please redirect me to the correct place if I've made a mistake. >> >> When opening firefox or chrome, and using it for a good 7 seconds, it >> hangs in "uninterruptible sleep" and I recieve a "BUG" in dmesg. This >> doesn't occur when reverting this commit: >> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=48cf516f8c. >> Ive attached the output of decode_stacktrace.sh and the relevant dmesg >> log to this email. >> >> Thanks > >> BUG: unable to handle kernel NULL pointer dereference at 00000000000000e8 > > Thanks for the bug report! This is offset 232 and that matches > file->f_mapping as per pahole > pahole -C file ./vmlinux | grep f_mapping > struct address_space * f_mapping; /* 232 8 */ > > I thought that each file really has to have a mapping. But the following > should heal the issue and add an extra care. > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index f64733c23067..fc9d70a9fbd1 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -66,6 +66,8 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma) > { > if (vma_is_anonymous(vma)) > return __transparent_hugepage_enabled(vma); > + if (!vma->vm_file || !vma->vm_file->f_mapping) > + return false; > if (shmem_mapping(vma->vm_file->f_mapping) && shmem_huge_enabled(vma)) > return __transparent_hugepage_enabled(vma); >From what I see in code in mm/mmap.c, it seems if vma->vm_file is non-zero vma->vm_file->f_mapping may be assumed to be non-NULL; see unlink_file_vma() and __vma_link_file() for two examples, which both use the construct: file = vma->vm_file; if (file) { struct address_space *mapping = file->f_mapping; [ ... ] [ code that dereferences "mapping" without further checks ] } I see nothing wrong with your second check but a few extra instructions performed, but depending upon how often transparent_hugepage_enabled() is called there may be at least theoretical performance concerns. William Kucharski william.kucharski@xxxxxxxxxx
