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

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Message-ID: <04dccff9-87dd-4e97-a712-b62fa51b32f7@redhat.com>
Date: Thu, 22 Aug 2024 11:29:51 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.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 21.08.24 12:03, Qi Zheng wrote:
> 
> 
> On 2024/8/21 17:53, David Hildenbrand wrote:
>> On 21.08.24 11:51, Qi Zheng wrote:
>>>
>>>
>>> On 2024/8/21 17:41, David Hildenbrand wrote:
>>>> On 21.08.24 11:24, Qi Zheng wrote:
>>>>>
>>>>>
>>>>> On 2024/8/21 17:17, LEROY Christophe wrote:
>>>>>>
>>>>>>
>>>>>> Le 21/08/2024 à 10:18, Qi Zheng a écrit :
>>>>>>> In handle_pte_fault(), we may modify the vmf->pte after acquiring the
>>>>>>> vmf->ptl, so convert it to using pte_offset_map_maywrite_nolock().
>>>>>>> But
>>>>>>> since we already do the pte_same() check, so there is no need to get
>>>>>>> pmdval to do pmd_same() check, just pass NULL to pmdvalp parameter.
>>>>>>>
>>>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>>>>>>> ---
>>>>>>>       mm/memory.c | 9 +++++++--
>>>>>>>       1 file changed, 7 insertions(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>>>> index 93c0c25433d02..d3378e98faf13 100644
>>>>>>> --- a/mm/memory.c
>>>>>>> +++ b/mm/memory.c
>>>>>>> @@ -5504,9 +5504,14 @@ static vm_fault_t handle_pte_fault(struct
>>>>>>> vm_fault *vmf)
>>>>>>>                * pmd by anon khugepaged, since that takes mmap_lock in
>>>>>>> write
>>>>>>>                * mode; but shmem or file collapse to THP could still
>>>>>>> morph
>>>>>>>                * it into a huge pmd: just retry later if so.
>>>>>>> +         *
>>>>>>> +         * Use the maywrite version to indicate that vmf->pte
>>>>>>> will be
>>>>>>> +         * modified, but since we will use pte_same() to detect the
>>>>>>> +         * change of the pte entry, there is no need to get pmdval.
>>>>>>>                */
>>>>>>> -        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?

> 
> 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.

-- 
Cheers,

David / dhildenb