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

[Laurent Dufour]

On 22/03/2018 16:40, Matthew Wilcox wrote:
> On Thu, Mar 22, 2018 at 04:32:00PM +0100, Laurent Dufour wrote:
>> On 21/03/2018 23:46, Matthew Wilcox wrote:
>>> On Wed, Mar 21, 2018 at 02:45:44PM -0700, Yang Shi wrote:
>>>> Marking vma as deleted sounds good. The problem for my current approach is
>>>> the concurrent page fault may succeed if it access the not yet unmapped
>>>> section. Marking deleted vma could tell page fault the vma is not valid
>>>> anymore, then return SIGSEGV.
>>>>
>>>>> does not care; munmap will need to wait for the existing munmap operation
>>>>
>>>> Why mmap doesn't care? How about MAP_FIXED? It may fail unexpectedly, right?
>>>
>>> The other thing about MAP_FIXED that we'll need to handle is unmapping
>>> conflicts atomically.  Say a program has a 200GB mapping and then
>>> mmap(MAP_FIXED) another 200GB region on top of it.  So I think page faults
>>> are also going to have to wait for deleted vmas (then retry the fault)
>>> rather than immediately raising SIGSEGV.
>>
>> 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.
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.

> 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 ?