Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: [RFC] Page table sharing | Date | Tue, 19 Feb 2002 03:12:28 +0100 |
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On February 19, 2002 02:53 am, Linus Torvalds wrote: > On Tue, 19 Feb 2002, Daniel Phillips wrote: > > > > Somebody might read fault, changing an entry when we're in the middle of > > copying it and might might do a duplicated read fault. > > You're confusing the mm->mmap_sem with the page_table_lock. > > The mm semaphore is really a read-write semaphore, and yes, there can be > multiple faulters active at the same time readin gin pages. > > But the page_table_lock is 100% exclusive, and while you hold the > page_table_lock there is absolutely _not_ going to be any concurrent page > faulting.
Sure there can be, because we only hold the mm->page_table_lock for this, somebody could be faulting through another mm sharing the page table. For this reason I believe I have to look at the page table count, and unless it's one, I have to do some extra exclusion.
> (NOTE! Sure, there might be another mm that has the same pmd shared, but > that one is going to do an unshare before it actually touches anything in > the pmd, so it's NOT going to change the values in the original pmd).
Actually, I was planning to keep the tables shared, even through swapin/ swapout. The data remains valid for all mm's, whether it's in ram or in swap.
> So I'm personally convinced that the locking shouldn't be needed at all, > if you just make sure that you do things in the right order (that, of > course, might need some memory barriers, which had better be implied by > the atomic dec-and-test anyway).
You've convinced me that it can be considerably streamlined, which is great, but it can't all go, and even now there's some missing.
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