linux-kernel - Re: [RFC v6 PATCH 2/2] mm: mmap: zap pages with read mmap

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Message-ID: <04a22c49-fe30-63ac-c1b7-46a405c810e2@linux.alibaba.com>
Date:   Tue, 7 Aug 2018 18:51:45 -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 2/2] mm: mmap: zap pages with read mmap_sem in
 munmap



On 8/6/18 10:45 PM, Michal Hocko wrote:
> On Mon 06-08-18 15:19:06, Yang Shi wrote:
>>
>> On 8/6/18 1:52 PM, Michal Hocko wrote:
>>> On Mon 06-08-18 13:48:35, Yang Shi wrote:
>>>> On 8/6/18 1:41 PM, Michal Hocko wrote:
>>>>> On Mon 06-08-18 09:46:30, Yang Shi wrote:
>>>>>> On 8/6/18 2:40 AM, Michal Hocko wrote:
>>>>>>> On Fri 03-08-18 14:01:58, Yang Shi wrote:
>>>>>>>> On 8/3/18 2:07 AM, Michal Hocko wrote:
>>>>>>>>> On Fri 27-07-18 02:10:14, Yang Shi wrote:
>>>>> [...]
>>>>>>>>>> If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, they are
>>>>>>>>>> considered as special mappings. They will be dealt with before zapping
>>>>>>>>>> pages with write mmap_sem held. Basically, just update vm_flags.
>>>>>>>>> Well, I think it would be safer to simply fallback to the current
>>>>>>>>> implementation with these mappings and deal with them on top. This would
>>>>>>>>> make potential issues easier to bisect and partial reverts as well.
>>>>>>>> Do you mean just call do_munmap()? It sounds ok. Although we may waste some
>>>>>>>> cycles to repeat what has done, it sounds not too bad since those special
>>>>>>>> mappings should be not very common.
>>>>>>> VM_HUGETLB is quite spread. Especially for DB workloads.
>>>>>> Wait a minute. In this way, it sounds we go back to my old implementation
>>>>>> with special handling for those mappings with write mmap_sem held, right?
>>>>> Yes, I would really start simple and add further enhacements on top.
>>>> If updating vm_flags with read lock is safe in this case, we don't have to
>>>> do this. The only reason for this special handling is about vm_flags update.
>>> Yes, maybe you are right that this is safe. I would still argue to have
>>> it in a separate patch for easier review, bisectability etc...
>> Sorry, I'm a little bit confused. Do you mean I should have the patch
>> *without* handling the special case (just like to assume it is safe to
>> update vm_flags with read lock), then have the other patch on top of it,
>> which simply calls do_munmap() to deal with the special cases?
> Just skip those special cases in the initial implementation and handle
> each special case in its own patch on top.

Thanks. VM_LOCKED area will not be handled specially since it is easy to 
handle it, just follow what do_munmap does. The special cases will just 
handle VM_HUGETLB, VM_PFNMAP and uprobe mappings.