From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: [PATCHi v2] mm: do not drop unused pages when userfaultd is running Date: Tue, 3 Jul 2018 07:23:12 +0200 Message-ID: References: <20180702075049.9157-1-borntraeger@de.ibm.com> <20180702140638.eb3edfaa611ba9fa018f92eb@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <20180702140638.eb3edfaa611ba9fa018f92eb@linux-foundation.org> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Archive: List-Post: To: Andrew Morton Cc: linux-mm@kvack.org, linux-s390@vger.kernel.org, kvm@vger.kernel.org, Janosch Frank , David Hildenbrand , Cornelia Huck , linux-kernel@vger.kernel.org, Martin Schwidefsky , Andrea Arcangeli , Mike Rapoport List-ID: On 07/02/2018 11:06 PM, Andrew Morton wrote: > On Mon, 2 Jul 2018 09:50:49 +0200 Christian Borntraeger wrote: > >> KVM guests on s390 can notify the host of unused pages. This can result >> in pte_unused callbacks to be true for KVM guest memory. >> >> If a page is unused (checked with pte_unused) we might drop this page >> instead of paging it. This can have side-effects on userfaultd, when the >> page in question was already migrated: >> >> The next access of that page will trigger a fault and a user fault >> instead of faulting in a new and empty zero page. As QEMU does not >> expect a userfault on an already migrated page this migration will fail. >> >> The most straightforward solution is to ignore the pte_unused hint if a >> userfault context is active for this VMA. >> >> ... >> >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -64,6 +64,7 @@ >> #include >> #include >> #include >> +#include >> >> #include >> >> @@ -1481,7 +1482,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, >> set_pte_at(mm, address, pvmw.pte, pteval); >> } >> >> - } else if (pte_unused(pteval)) { >> + } else if (pte_unused(pteval) && !userfaultfd_armed(vma)) { >> /* >> * The guest indicated that the page content is of no >> * interest anymore. Simply discard the pte, vmscan > > A reader of this code will wonder why we're checking > userfaultfd_armed(). So the writer of this code should add a comment > which explains this to them ;) Please. > Something like: /* * The guest indicated that the page content is of no * interest anymore. Simply discard the pte, vmscan * will take care of the rest. * A future reference will then fault in a new zero * page. When userfaultfd is active, we must not drop * this page though, as its main user (postcopy * migration) will not expect userfaults on already * copied pages. */ ?