linux-kernel - Re: [PATCH 06/14] mm: handle_pte_fault() use pte_offset_map_maywrite

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Message-ID: <7e83f87b-703b-4f7c-9d42-78845acb41c6@bytedance.com>
Date: Thu, 22 Aug 2024 20:22:37 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: David Hildenbrand <david@...hat.com>
Cc: LEROY Christophe <christophe.leroy2@...soprasteria.com>,
 "hughd@...gle.com" <hughd@...gle.com>,
 "willy@...radead.org" <willy@...radead.org>,
 "muchun.song@...ux.dev" <muchun.song@...ux.dev>,
 "vbabka@...nel.org" <vbabka@...nel.org>,
 "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
 "rppt@...nel.org" <rppt@...nel.org>,
 "vishal.moola@...il.com" <vishal.moola@...il.com>,
 "peterx@...hat.com" <peterx@...hat.com>,
 "ryan.roberts@....com" <ryan.roberts@....com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 06/14] mm: handle_pte_fault() use
 pte_offset_map_maywrite_nolock()



On 2024/8/22 20:19, David Hildenbrand wrote:
> On 22.08.24 14:17, Qi Zheng wrote:
>> Hi David,
>>
>> On 2024/8/22 17:29, David Hildenbrand wrote:
>>> On 21.08.24 12:03, Qi Zheng wrote:
>>>>
>>>>
>>
>> [...]
>>
>>>>>>>>>> -        vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm,
>>>>>>>>>> vmf->pmd,
>>>>>>>>>> -                         vmf->address, &vmf->ptl);
>>>>>>>>>> +        vmf->pte = 
>>>>>>>>>> pte_offset_map_maywrite_nolock(vmf->vma->vm_mm,
>>>>>>>>>> +                              vmf->pmd, vmf->address,
>>>>>>>>>> +                              NULL, &vmf->ptl);
>>>>>>>
>>>>>>> I think we discussed that passing NULL should be forbidden for that
>>>>>>> function.
>>>>>>
>>>>>> Yes, but for some maywrite case, there is no need to get pmdval to
>>>>>> do pmd_same() check. So I passed NULL and added a comment to
>>>>>> explain this.
>>>>>
>>>>> I wonder if it's better to pass a dummy variable instead. One has to
>>>>> think harder why that is required compared to blindly passing 
>>>>> "NULL" :)
>>>>
>>>> You are afraid that subsequent caller will abuse this function, right?
>>>
>>> Yes! "oh, I don't need a pmdval, why would I? let's just pass NULL,
>>> easy" :)
>>>
>>>> My initial concern was that this would add a useless local vaiable, but
>>>> perhaps that is not a big deal.
>>>
>>> How many of these "special" instances do we have?
>>
>> We have 5 such special instances.
>>
>>>
>>>>
>>>> Both are fine for me. ;)
>>>
>>> Also no strong opinion, but having to pass a variable makes you think
>>> what you are supposed to do with it and why it is not optional.
>>
>> Yeah, I added 'BUG_ON(!pmdvalp);' in pte_offset_map_ro_nolock(), and
>> have updated the v2 version [1].
> 
> No BUG_ON please :) VM_WARN_ON_ONCE() is good enough.

Got it. Will do in the next version.

>