linux-kernel - Re: [RFC v6 PATCH 1/2] mm: refactor do_munmap() to extract the common part

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Message-ID: <6af33f8f-042a-888f-2dad-6023fa5533f0@linux.alibaba.com>
Date:   Mon, 6 Aug 2018 09:53:13 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     willy@...radead.org, ldufour@...ux.vnet.ibm.com,
        kirill@...temov.name, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v6 PATCH 1/2] mm: refactor do_munmap() to extract the common
 part



On 8/6/18 6:26 AM, Michal Hocko wrote:
> On Fri 03-08-18 13:47:19, Yang Shi wrote:
>>
>> On 8/3/18 1:53 AM, Michal Hocko wrote:
>>> On Fri 27-07-18 02:10:13, Yang Shi wrote:
>>>> Introduces three new helper functions:
>>>>     * munmap_addr_sanity()
>>>>     * munmap_lookup_vma()
>>>>     * munmap_mlock_vma()
>>>>
>>>> They will be used by do_munmap() and the new do_munmap with zapping
>>>> large mapping early in the later patch.
>>>>
>>>> There is no functional change, just code refactor.
>>>>
>>>> Reviewed-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
>>>> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
>>>> ---
>>>>    mm/mmap.c | 120 ++++++++++++++++++++++++++++++++++++++++++--------------------
>>>>    1 file changed, 82 insertions(+), 38 deletions(-)
>>>>
>>>> diff --git a/mm/mmap.c b/mm/mmap.c
>>>> index d1eb87e..2504094 100644
>>>> --- a/mm/mmap.c
>>>> +++ b/mm/mmap.c
>>>> @@ -2686,34 +2686,44 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
>>>>    	return __split_vma(mm, vma, addr, new_below);
>>>>    }
>>>> -/* Munmap is split into 2 main parts -- this part which finds
>>>> - * what needs doing, and the areas themselves, which do the
>>>> - * work.  This now handles partial unmappings.
>>>> - * Jeremy Fitzhardinge <jeremy@...p.org>
>>>> - */
>>>> -int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
>>>> -	      struct list_head *uf)
>>>> +static inline bool munmap_addr_sanity(unsigned long start, size_t len)
>>> munmap_check_addr? Btw. why does this need to have munmap prefix at all?
>>> This is a general address space check.
>> Just because I extracted this from do_munmap, no special consideration. It
>> is definitely ok to use another name.
>>
>>>>    {
>>>> -	unsigned long end;
>>>> -	struct vm_area_struct *vma, *prev, *last;
>>>> -
>>>>    	if ((offset_in_page(start)) || start > TASK_SIZE || len > TASK_SIZE-start)
>>>> -		return -EINVAL;
>>>> +		return false;
>>>> -	len = PAGE_ALIGN(len);
>>>> -	if (len == 0)
>>>> -		return -EINVAL;
>>>> +	if (PAGE_ALIGN(len) == 0)
>>>> +		return false;
>>>> +
>>>> +	return true;
>>>> +}
>>>> +
>>>> +/*
>>>> + * munmap_lookup_vma: find the first overlap vma and split overlap vmas.
>>>> + * @mm: mm_struct
>>>> + * @vma: the first overlapping vma
>>>> + * @prev: vma's prev
>>>> + * @start: start address
>>>> + * @end: end address
>>> This really doesn't help me to understand how to use the function.
>>> Why do we need both prev and vma etc...
>> prev will be used by unmap_region later.
> But what does it stand for? Why cannot you take prev from the returned
> vma? In other words, if somebody reads this documentation how does he
> know what the prev is supposed to be used for?
>
>>>> + *
>>>> + * returns 1 if successful, 0 or errno otherwise
>>> This is a really weird calling convention. So what does 0 tell? /me
>>> checks the code. Ohh, it is nothing to do. Why cannot you simply return
>>> the vma. NULL implies nothing to do, ERR_PTR on error.
>> A couple of reasons why it is implemented as so:
>>
>>      * do_munmap returns 0 for both success and no suitable vma
>>
>>      * Since prev is needed by finding the start vma, and prev will be used
>> by unmap_region later too, so I just thought it would look clean to have one
>> function to return both start vma and prev. In this way, we can share as
>> much as possible common code.
>>
>>      * In this way, we just need return 0, 1 or error no just as same as what
>> do_munmap does currently. Then we know what is failure case exactly to just
>> bail out right away.
>>
>> Actually, I tried the same approach as you suggested, but it had two
>> problems:
>>
>>      * If it returns the start vma, we have to re-find its prev later, but
>> the prev has been found during finding start vma. And, duplicate the code in
>> do_munmap_zap_rlock. It sounds not that ideal.
>>
>>      * If it returns prev, it might be null (start vma is the first vma). We
>> can't tell if null is a failure or success case
> Even if you need to return both vma and prev then it would be better to
> simply return vma directly than having this -errno, 0 or 1 return
> semantic.

OK, I will try to refactor the code.

Thanks,
Yang