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 17:35:57 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....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 2022/2/15 16:37, HORIGUCHI NAOYA(堀口 直也) wrote:
> 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?

Agree. anon_vma_interval_tree_foreach/vma_interval_tree_foreach in collect_procs_{file,anon} should
have guaranteed the validity of the vma_address(). And rmap_walk_anon and rmap_walk_file do the
VM_BUG_ON_VMA(address == -EFAULT, vma). So VM_BUG_ON() might be really more suitable. Will do this
in v2.

Many thanks.

> 
> Thanks,
> Naoya Horiguchi
>