Re: [RFC v6 PATCH 2/2] mm: mmap: zap pages with read mmap_sem in munmap

[Yang Shi <yang.shi@linux.alibaba.com>]

On 8/8/18 2:22 AM, Vlastimil Babka wrote:
> On 08/08/2018 03:51 AM, Yang Shi wrote:
>> 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.
> So I think you could maybe structure code like this: instead of
> introducing do_munmap_zap_rlock() and all those "bool skip_vm_flags"
> additions, add a boolean parameter in do_munmap() to use the new
> behavior, with only the first user SYSCALL_DEFINE2(munmap) setting it to
> true. If true, do_munmap() will do the
> - down_write_killable() itself instead of assuming it's already locked
> - munmap_lookup_vma()
> - check if any of the vma's in the range is "special", if yes, change
> the boolean param to "false", and continue like previously, e.g. no mmap
> sem downgrade etc.

Thanks for the suggestion. Actually, I did the similar thing in v1 
patches, which added a bool parameter in vm_munmap() to tell if 
releasing mmap_sem is acceptable for some code paths. But, it got pushed 
back by tglx since vm_munmap() is called by x86 specific code too (and 
some other architectures). He suggested to define a new function to do 
the optimization. So, I followed this approach in the later versions.

Yang

>
> That would be a basis for further optimizing the special vma cases in
> subsequent patches (maybe it's really ok to touch the vma flags with
> mmap sem for read as vma's are detached), and to eventually convert more
> do_munmap() callers to the new mode.
>
> HTH,
> Vlastimil
>
>
>