On Thu, Oct 10, 2013 at 10:16:46PM +0300, Gleb Natapov wrote: > On Thu, Oct 10, 2013 at 01:42:22PM -0300, Marcelo Tosatti wrote: > > On Thu, Oct 10, 2013 at 03:08:45PM +0300, Gleb Natapov wrote: > > > On Wed, Oct 09, 2013 at 10:47:10PM -0300, Marcelo Tosatti wrote: > > > > > >> Gleb has a idea that uses RCU_DESTORY to protect the shadow page table > > > > > >> and encodes the page-level into the spte (since we need to check if the spte > > > > > >> is the last-spte. ). How about this? > > > > > > > > > > > > Pointer please? Why is DESTROY_SLAB_RCU any safer than call_rcu with > > > > > > regards to limitation? (maybe it is). > > > > > > > > > > For my experience, freeing shadow page and allocing shadow page are balanced, > > > > > we can check it by (make -j12 on a guest with 4 vcpus and): > > > > > > > > > > # echo > trace > > > > > [root@eric-desktop tracing]# cat trace > ~/log | sleep 3 > > > > > [root@eric-desktop tracing]# cat ~/log | grep new | wc -l > > > > > 10816 > > > > > [root@eric-desktop tracing]# cat ~/log | grep prepare | wc -l > > > > > 10656 > > > > > [root@eric-desktop tracing]# cat set_event > > > > > kvmmmu:kvm_mmu_get_page > > > > > kvmmmu:kvm_mmu_prepare_zap_page > > > > > > > > > > alloc VS. free = 10816 : 10656 > > > > > > > > > > So that, mostly all allocing and freeing are done in the slab's > > > > > cache and the slab frees shdadow pages very slowly, there is no rcu issue. > > > > > > > > A more detailed test case would be: > > > > > > > > - cpu0-vcpu0 releasing pages as fast as possible > > > > - cpu1 executing get_dirty_log > > > > > > > > Think of a very large guest. > > > > > > > The number of shadow pages allocated from slab will be bounded by > > > n_max_mmu_pages, > > > > Correct, but that limit is not suitable (maximum number of mmu pages > > should be larger than number of mmu pages freeable in a rcu grace > > period). > > > I am not sure I understand what you mean here. What I was sating is that if > we change code to allocate sp->spt from slab, this slab will never have > more then n_max_mmu_pages objects in it. n_max_mmu_pages is not a suitable limit to throttle freeing of pages via RCU (its too large). If the free memory watermarks are smaller than n_max_mmu_pages for all guests, OOM is possible. > > > and, in addition, page released to slab is immediately > > > available for allocation, no need to wait for grace period. > > > > See SLAB_DESTROY_BY_RCU comment at include/linux/slab.h. > > > This comment is exactly what I was referring to in the code you quoted. Do > you see anything problematic in what comment describes? "This delays freeing the SLAB page by a grace period, it does _NOT_ delay object freeing." The page is not available for allocation. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
