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 09:49:01 -0700 |
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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
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