Messages in this thread | | | Subject | Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section | From | Yang Shi <> | Date | Thu, 22 Mar 2018 09:46:38 -0700 |
| |
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'm not sure if completion can do this or not since I'm not quite familiar with it :-(
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.
| |