From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared pages To: "Kirill A. Shutemov" , Michal Hocko Cc: kbuild test robot , kbuild-all@01.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A . Shutemov" , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Vlastimil Babka , Naoya Horiguchi , Davidlohr Bueso , Andrew Morton , stable@vger.kernel.org References: <20180821205902.21223-2-mike.kravetz@oracle.com> <201808220831.eM0je51n%fengguang.wu@intel.com> <975b740d-26a6-eb3f-c8ca-1a9995d0d343@oracle.com> <20180822122848.GL29735@dhcp22.suse.cz> <4a95a24f-534f-0938-f358-2a410817a412@oracle.com> <20180823073035.GT29735@dhcp22.suse.cz> <20180823082112.xln7rinqcwt54teg@kshutemo-mobl1> From: Mike Kravetz Message-ID: Date: Thu, 23 Aug 2018 09:45:17 -0700 MIME-Version: 1.0 In-Reply-To: <20180823082112.xln7rinqcwt54teg@kshutemo-mobl1> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: On 08/23/2018 01:21 AM, Kirill A. Shutemov wrote: > On Thu, Aug 23, 2018 at 09:30:35AM +0200, Michal Hocko wrote: >> On Wed 22-08-18 09:48:16, Mike Kravetz wrote: >>> On 08/22/2018 05:28 AM, Michal Hocko wrote: >>>> On Tue 21-08-18 18:10:42, Mike Kravetz wrote: >>>> [...] >>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>> index eb477809a5c0..8cf853a4b093 100644 >>>>> --- a/mm/rmap.c >>>>> +++ b/mm/rmap.c >>>>> @@ -1362,11 +1362,21 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, >>>>> } >>>>> >>>>> /* >>>>> - * We have to assume the worse case ie pmd for invalidation. Note that >>>>> - * the page can not be free in this function as call of try_to_unmap() >>>>> - * must hold a reference on the page. >>>>> + * For THP, we have to assume the worse case ie pmd for invalidation. >>>>> + * For hugetlb, it could be much worse if we need to do pud >>>>> + * invalidation in the case of pmd sharing. >>>>> + * >>>>> + * Note that the page can not be free in this function as call of >>>>> + * try_to_unmap() must hold a reference on the page. >>>>> */ >>>>> end = min(vma->vm_end, start + (PAGE_SIZE << compound_order(page))); >>>>> + if (PageHuge(page)) { >>>>> + /* >>>>> + * If sharing is possible, start and end will be adjusted >>>>> + * accordingly. >>>>> + */ >>>>> + (void)huge_pmd_sharing_possible(vma, &start, &end); >>>>> + } >>>>> mmu_notifier_invalidate_range_start(vma->vm_mm, start, end); >>>> >>>> I do not get this part. Why don't we simply unconditionally invalidate >>>> the whole huge page range? >>> >>> In this routine, we are only unmapping a single page. The existing code >>> is limiting the invalidate range to that page size: 4K or 2M. With shared >>> PMDs, we have the possibility of unmapping a PUD_SIZE area: 1G. I don't >>> think we want to unconditionally invalidate 1G. Is that what you are asking? >> >> But we know that huge_pmd_unshare unmapped a shared pte so we know when >> to flush 2MB or 1GB. I really do not like how huge_pmd_sharing_possible >> a) duplicates some checks and b) it updates start/stop out of line. > > My reading on this is that mmu_notifier_invalidate_range_start() has to be > called from sleepable context on the full range that *can* be invalidated > before following mmu_notifier_invalidate_range_end(). > > In this case huge_pmd_unshare() may unmap aligned PUD_SIZE around the PMD > page that effectively enlarge range that has to be covered by > mmu_notifier_invalidate_range_start(). We cannot yet know if there's any > shared page tables in the range, so we need to go with worst case > scenario. Yes, that is a good summary. We can not know for sure if there is PMD sharing until we hold the page table lock. So, we don't know if we should invalidate/flush 2M or 1G. Yet, we need to call mmu_notifier_invalidate_range_start before taking the lock. And, the notifiers need to know the range of the worst possible case. The best approach I came up with is to adjust the range if sharing is 'possible'. > > I don't see conceptually better solution than what is proposed. > I have updated the patches based on Kirill's previous comments and will send out a new version later today. -- Mike Kravetz