From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ua1-f71.google.com (mail-ua1-f71.google.com [209.85.222.71]) by kanga.kvack.org (Postfix) with ESMTP id 62A666B2543 for ; Wed, 22 Aug 2018 12:48:31 -0400 (EDT) Received: by mail-ua1-f71.google.com with SMTP id g9-v6so942589uam.17 for ; Wed, 22 Aug 2018 09:48:31 -0700 (PDT) Received: from aserp2120.oracle.com (aserp2120.oracle.com. [141.146.126.78]) by mx.google.com with ESMTPS id b25-v6si874766uap.322.2018.08.22.09.48.28 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 22 Aug 2018 09:48:28 -0700 (PDT) Subject: Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared pages 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> From: Mike Kravetz Message-ID: <4a95a24f-534f-0938-f358-2a410817a412@oracle.com> Date: Wed, 22 Aug 2018 09:48:16 -0700 MIME-Version: 1.0 In-Reply-To: <20180822122848.GL29735@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: 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 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? I do not know how often PMD sharing is exercised. It certainly is used by DBs for large shared areas. I suspect it is less frequent than hugtlb pages in general, and certainly less frequent than THP or base pages. >> >> while (page_vma_mapped_walk(&pvmw)) { >> @@ -1409,6 +1419,32 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, >> subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte); >> address = pvmw.address; >> >> + if (PageHuge(page)) { >> + if (huge_pmd_unshare(mm, &address, pvmw.pte)) { > > huge_pmd_unshare is documented to require a pte lock. Where do we take > it? It is somewhat hidden, but we are in the loop: while (page_vma_mapped_walk(&pvmw)) { The routine page_vma_mapped_walk will acquire the lock, and it correctly checks for huge pages and uses huge_pte_lockptr(). page_vma_mapped_walk_done() will release the lock. -- Mike Kravetz