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[91.12.97.4]) by smtp.gmail.com with ESMTPSA id e6sm5879095wme.27.2020.11.20.14.06.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Nov 2020 14:06:06 -0800 (PST) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable From: David Hildenbrand Mime-Version: 1.0 (1.0) Subject: Re: Pinning ZONE_MOVABLE pages Date: Fri, 20 Nov 2020 23:06:05 +0100 Message-Id: References: Cc: David Hildenbrand , linux-mm , Andrew Morton , Vlastimil Babka , LKML , Michal Hocko , Oscar Salvador , Dan Williams , Sasha Levin , Tyler Hicks , Joonsoo Kim , sthemmin@microsoft.com In-Reply-To: To: Pavel Tatashin X-Mailer: iPhone Mail (18A8395) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Am 20.11.2020 um 22:58 schrieb Pavel Tatashin := >=20 > =EF=BB=BFOn Fri, Nov 20, 2020 at 3:59 PM David Hildenbrand wrote: >>=20 >>=20 >>>> Am 20.11.2020 um 21:28 schrieb Pavel Tatashin : >>>=20 >>> =EF=BB=BFRecently, I encountered a hang that is happening during memory h= ot >>> remove operation. It turns out that the hang is caused by pinned user >>> pages in ZONE_MOVABLE. >>>=20 >>> Kernel expects that all pages in ZONE_MOVABLE can be migrated, but >>> this is not the case if a user applications such as through dpdk >>> libraries pinned them via vfio dma map. Kernel keeps trying to >>> hot-remove them, but refcnt never gets to zero, so we are looping >>> until the hardware watchdog kicks in. >>>=20 >>> We cannot do dma unmaps before hot-remove, because hot-remove is a >>> slow operation, and we have thousands for network flows handled by >>> dpdk that we just cannot suspend for the duration of hot-remove >>> operation. >>>=20 >>=20 >> Hi! >>=20 >> It=E2=80=98s a known problem also for VMs using vfio. I thought about thi= s some while ago an came to the same conclusion: before performing long-term= pinnings, we have to migrate pages off the movable zone. After that, it=E2=80= =98s too late. >>=20 >> What happens when we can=E2=80=98t migrate (OOM on !MOVABLE memory, short= -term pinning)? TBD. >>=20 >>> The solution is for dpdk to allocate pages from a zone below >>> ZONE_MOVAVLE, i.e. ZONE_NORMAL/ZONE_HIGHMEM, but this is not possible. >>> There is no user interface that we have that allows applications to >>> select what zone the memory should come from. >>>=20 >>> I've spoken with Stephen Hemminger, and he said that DPDK is moving in >>> the direction of using transparent huge pages instead of HugeTLBs, >>> which means that we need to allow at least anonymous, and anonymous >>> transparent huge pages to come from non-movable zones on demand. >>>=20 >>> Here is what I am proposing: >>> 1. Add a new flag that is passed through pin_user_pages_* down to >>> fault handlers, and allow the fault handler to allocate from a >>> non-movable zone. >>>=20 >>> Sample function stacks through which this info needs to be passed is thi= s: >>>=20 >>> pin_user_pages_remote(gup_flags) >>> __get_user_pages_remote(gup_flags) >>> __gup_longterm_locked(gup_flags) >>> __get_user_pages_locked(gup_flags) >>> __get_user_pages(gup_flags) >>> faultin_page(gup_flags) >>> Convert gup_flags into fault_flags >>> handle_mm_fault(fault_flags) >>>=20 >>> =46rom handle_mm_fault(), the stack diverges into various faults, >>> examples include: >>>=20 >>> Transparent Huge Page >>> handle_mm_fault(fault_flags) >>> __handle_mm_fault(fault_flags) >>> Create: struct vm_fault vmf, use fault_flags to specify correct gfp_mask= >>> create_huge_pmd(vmf); >>> do_huge_pmd_anonymous_page(vmf); >>> mm_get_huge_zero_page(vma->vm_mm); -> flag is lost, so flag from >>> vmf.gfp_mask should be passed as well. >>>=20 >>> There are several other similar paths in a transparent huge page, also >>> there is a named path where allocation is based on filesystems, and >>> the flag should be honored there as well, but it does not have to be >>> added at the same time. >>>=20 >>> Regular Pages >>> handle_mm_fault(fault_flags) >>> __handle_mm_fault(fault_flags) >>> Create: struct vm_fault vmf, use fault_flags to specify correct gfp_mask= >>> handle_pte_fault(vmf) >>> do_anonymous_page(vmf); >>> page =3D alloc_zeroed_user_highpage_movable(vma, vmf->address); -> >>> replace change this call according to gfp_mask. >>>=20 >>> The above only take care of the case if user application faults on the >>> page during pinning time, but there are also cases where pages already >>> exist. >>>=20 >>> 2. Add an internal move_pages_zone() similar to move_pages() syscall >>> but instead of migrating to a different NUMA node, migrate pages from >>> ZONE_MOVABLE to another zone. >>> Call move_pages_zone() on demand prior to pinning pages from >>> vfio_pin_map_dma() for instance. >>>=20 >>> 3. Perhaps, it also makes sense to add madvise() flag, to allocate >>> pages from non-movable zone. When a user application knows that it >>> will do DMA mapping, and pin pages for a long time, the memory that it >>> allocates should never be migrated or hot-removed, so make sure that >>> it comes from the appropriate place. >>> The benefit of adding madvise() flag is that we won't have to deal >>> with slow page migration during pin time, but the disadvantage is that >>> we would need to change the user interface. >>>=20 >>=20 >> Hm, I am not sure we want to expose these details. What would be the sema= ntics? =E2=80=9EMight pin=E2=80=9C? Hm, not sure. >=20 > The semantic would be PA must not change, something that DPDK > currently excpects from huge pages, which by the way is not true, as > huge pages are migratable. >=20 >>=20 >> Assume you start a fresh VM via QEMU with vfio. When we start mapping gue= st memory via vfio, that=E2=80=98s usually the time memory will get populate= d. Not really much has to be migrated. I think this is even true during live= migration. >>=20 >> I think selective DMA pinning (e.g., vIOMMU in QEMU) is different, where w= e keep pinning/unpinning on demand. But I guess even here, we will often reu= se some pages over and over again. >>=20 >>=20 >>> Before I start working on the above approaches, I would like to get an >>> opinion from the community on an appropriate path forward for this >>> problem. If what I described sounds reasonable, or if there are other >>> ideas on how to address the problem that I am seeing. >>=20 >> At least 1 and 2 sound sane. 3 is TBD - but it=E2=80=98s a pure optimizat= ion, so it can wait. >=20 > Makes sense, I am also worried about 3, but most of madvise() flags > are for pure optimization purposes: MADV_HUGEPAGE, MADV_SEQUENTIAL, > MADV_WILLNEED etc. BTW, I assume we should also directly tackle migrating pages off CMA regions= when pinning, I guess quite some people will be interested in that as well.= Have a nice weekend and thanks for looking into this issue :)=