On 11/28/2013 04:53 PM, Xiao Guangrong wrote: > On 11/27/2013 03:31 AM, Marcelo Tosatti wrote: >> On Tue, Nov 26, 2013 at 11:21:37AM +0800, Xiao Guangrong wrote: >>> On 11/26/2013 02:12 AM, Marcelo Tosatti wrote: >>>> On Mon, Nov 25, 2013 at 02:29:03PM +0800, Xiao Guangrong wrote: >>>>>>> Also, there is no guarantee of termination (as long as sptes are >>>>>>> deleted with the correct timing). BTW, can't see any guarantee of >>>>>>> termination for rculist nulls either (a writer can race with a lockless >>>>>>> reader indefinately, restarting the lockless walk every time). >>>>>> >>>>>> Hmm, that can be avoided by checking dirty-bitmap before rewalk, >>>>>> that means, if the dirty-bitmap has been set during lockless >>>>>> write-protection, >>>>>> it�s unnecessary to write-protect its sptes. Your idea? >>>>> This idea is based on the fact that the number of rmap is limited by >>>>> RMAP_RECYCLE_THRESHOLD. So, in the case of adding new spte into rmap, >>>>> we can break the rewalk at once, in the case of deleting, we can only >>>>> rewalk RMAP_RECYCLE_THRESHOLD times. >>>> >>>> Please explain in more detail. >>> >>> Okay. >>> >>> My proposal is like this: >>> >>> pte_list_walk_lockless() >>> { >>> restart: >>> >>> + if (__test_bit(slot->arch.dirty_bitmap, gfn-index)) >>> + return; >>> >>> code-doing-lockless-walking; >>> ...... >>> } >>> >>> Before do lockless-walking, we check the dirty-bitmap first, if >>> it is set we can simply skip write-protection for the gfn, that >>> is the case that new spte is being added into rmap when we lockless >>> access the rmap. >> >> The dirty bit could be set after the check. >> >>> For the case of deleting spte from rmap, the number of entry is limited >>> by RMAP_RECYCLE_THRESHOLD, that is not endlessly. >> >> It can shrink and grow while lockless walk is performed. > > Yes, indeed. > > Hmmm, another idea in my mind to fix this is encoding the position into > the reserved bits of desc->more pointer, for example: > > +------+ +------+ +------+ > rmapp -> |Desc 0| -> |Desc 1| -> |Desc 2| > +------+ +------+ +------+ > > There are 3 descs on the rmap, and: > rmapp = &desc0 | 1UL | 3UL << 50; > desc0->more = desc1 | 2UL << 50; > desc1->more = desc0 | 1UL << 50 > desc2->more = &rmapp | 1UL; (The nulls pointer) > > We will walk to the next desc only if the "position" of current desc > is >= the position of next desc. That can make sure we can reach the > last desc anyway. > > And in order to avoiding doing too many "rewalk", we will goto the > slow path (do walk with holding the lock) instead when retry the walk > more that N times.
How about this idea? Or you guys still prefer to the idea of lockless on first-level? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/