Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section

[Laurent Dufour]

On 22/03/2018 17:46, Yang Shi wrote:
> 
> 
> On 3/22/18 9:18 AM, Laurent Dufour wrote:
>>
>> On 22/03/2018 17:05, Matthew Wilcox wrote:
>>> On Thu, Mar 22, 2018 at 04:54:52PM +0100, Laurent Dufour wrote:
>>>> On 22/03/2018 16:40, Matthew Wilcox wrote:
>>>>> On Thu, Mar 22, 2018 at 04:32:00PM +0100, Laurent Dufour wrote:
>>>>>> Regarding the page fault, why not relying on the PTE locking ?
>>>>>>
>>>>>> When munmap() will unset the PTE it will have to held the PTE lock, so this
>>>>>> will serialize the access.
>>>>>> If the page fault occurs before the mmap(MAP_FIXED), the page mapped will be
>>>>>> removed when mmap(MAP_FIXED) would do the cleanup. Fair enough.
>>>>> The page fault handler will walk the VMA tree to find the correct
>>>>> VMA and then find that the VMA is marked as deleted. If it assumes
>>>>> that the VMA has been deleted because of munmap(), then it can raise
>>>>> SIGSEGV immediately. But if the VMA is marked as deleted because of
>>>>> mmap(MAP_FIXED), it must wait until the new VMA is in place.
>>>> I'm wondering if such a complexity is required.
>>>> If the user space process try to access the page being overwritten through
>>>> mmap(MAP_FIXED) by another thread, there is no guarantee that it will
>>>> manipulate the *old* page or *new* one.
>>> Right; but it must return one or the other, it can't segfault.
>> Good point, I missed that...
>>
>>>> I'd think this is up to the user process to handle that concurrency.
>>>> What needs to be guaranteed is that once mmap(MAP_FIXED) returns the old page
>>>> are no more there, which is done through the mmap_sem and PTE locking.
>>> Yes, and allowing the fault handler to return the *old* page risks the
>>> old page being reinserted into the page tables after the unmapping task
>>> has done its work.
>> The PTE locking should prevent that.
>>
>>> It's *really* rare to page-fault on a VMA which is in the middle of
>>> being replaced. Why are you trying to optimise it?
>> I was not trying to optimize it, but to not wait in the page fault handler.
>> This could become tricky in the case the VMA is removed once mmap(MAP_FIXED) is
>> done and before the waiting page fault got woken up. This means that the
>> removed VMA structure will have to remain until all the waiters are woken up
>> which implies ref_count or similar.
> 
> We may not need ref_count. After removing "locked-for-deletion" vmas when
> mmap(MAP_FIXED) is done, just wake up page fault to re-lookup vma, then it will
> find the new vma installed by mmap(MAP_FIXED), right?

I do agree, as far as waking up would not require access to the VMA.

> I'm not sure if completion can do this or not since I'm not quite familiar with
> it :-(

I don't know either :/

Laurent.

> Yang
> 
>>
>>>>> I think I was wrong to describe VMAs as being *deleted*. I think we
>>>>> instead need the concept of a *locked* VMA that page faults will block on.
>>>>> Conceptually, it's a per-VMA rwsem, but I'd use a completion instead of
>>>>> an rwsem since the only reason to write-lock the VMA is because it is
>>>>> being deleted.
>>>> Such a lock would only makes sense in the case of mmap(MAP_FIXED) since when
>>>> the VMA is removed there is no need to wait. Isn't it ?
>>> I can't think of another reason. I suppose we could mark the VMA as
>>> locked-for-deletion or locked-for-replacement and have the SIGSEGV happen
>>> early. But I'm not sure that optimising for SIGSEGVs is a worthwhile
>>> use of our time. Just always have the pagefault sleep for a deleted VMA.
> 
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