On Tue, 1 Feb 2005, Nick Piggin wrote: > > pte_unmap(page_table); > > + page_table_atomic_stop(mm); > > > > /* > > * Ok, we need to copy. Oh, well.. > > */ > > if (!PageReserved(old_page)) > > page_cache_get(old_page); > > - spin_unlock(&mm->page_table_lock); > > > > I don't think you can do this unless you have done something funky that I > missed. And that kind of shoots down your lockless COW too, although it > looks like you can safely have the second part of do_wp_page without the > lock. Basically - your lockless COW patch itself seems like it should be > OK, but this hunk does not.
See my comment at the end of this message. > I would be very interested if you are seeing performance gains with your > lockless COW patches, BTW. So far I have not had time to focus on benchmarking that. > Basically, getting a reference on a struct page was the only thing I found > I wasn't able to do lockless with pte cmpxchg. Because it can race with > unmapping in rmap.c and reclaim and reuse, which probably isn't too good. > That means: the only operations you are able to do lockless is when there > is no backing page (ie. the anonymous unpopulated->populated case). > > A per-pte lock is sufficient for this case, of course, which is why the > pte-locked system is completely free of the page table lock. Introducing pte locking would allow us to go further with parallelizing this but its another invasive procedure. I think parallelizing COW is only possible to do reliable with some pte locking scheme. But then the question is if the pte locking is really faster than obtaining a spinlock. I suspect this may not be the case. > Although I may have some fact fundamentally wrong? The unmapping in rmap.c would change the pte. This would be discovered after acquiring the spinlock later in do_wp_page. Which would then lead to the operation being abandoned. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
