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

[Yang Shi]

On 3/22/18 9:05 AM, 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.
>
> > 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.

It sounds worth to me. If we have every page fault sleep to wait for vma 
deletion is done, it sounds equal to wait for mmap_sem all the time, right?

Yang