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

[Matthew Wilcox]

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.

> 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.

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 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.