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

[Laurent Dufour <ldufour@linux.vnet.ibm.com>]

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.