Re: [PATCH 2/8] mm/memory-failure.c: avoid walking page table when vma_address() return -EFAULT

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Tue, Feb 15, 2022 at 10:40:02AM +0800, Miaohe Lin wrote:
> On 2022/2/14 22:48, Naoya Horiguchi wrote:
> > On Thu, Feb 10, 2022 at 10:17:27PM +0800, Miaohe Lin wrote:
> >> It's unnecessary to walk the page table when vma_address() return -EFAULT.
> >> Return early if so to save some cpu cycles.
> >>
> >> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> > 
> > Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> 
> Many thanks for your review and Acked-by tag!

You're welcome :)

> 
> > 
> > Does this patch fix the real problem rather than just saving cpu cycles?
> > Without this patch, "address == -EFAULT" seems to make pgd_offset() return
> > invalid pointer and result in some serious result like general protection fault.
> 
> I think you're right. We might dereference the invalid pointer in the following pagetable
> walk and results in general protection fault.
> 
> > If that's the case, this patch might be worth sending to stable.
> 
> But I'am not sure vma_address will return -EFAULT for dax pages in the real workload?
> If so, I will send a v2 with Fixes tag.

Hm, actually I'm not sure either.  But dev_pagemap_mapping_shift() is called only
when vma associated to the error page is found already in collect_procs_{file,anon},
so vma_address() should not return -EFAULT except with some bug.
So VM_BUG_ON() might be more suitable?

Thanks,
Naoya Horiguchi