linux-kernel - Re: [PATCH 2/8] mm/memory-failure.c: avoid walking page table when vma

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Date:   Tue, 15 Feb 2022 08:37:56 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     Miaohe Lin <linmiaohe@...wei.com>
CC:     Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/8] mm/memory-failure.c: avoid walking page table when
 vma_address() return -EFAULT

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@...wei.com>
> > 
> > Acked-by: Naoya Horiguchi <naoya.horiguchi@....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