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Wed, 11 Dec 2019 07:07:34 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id EB609DB32; Wed, 11 Dec 2019 12:07:32 +0000 (UTC) Received: from [10.36.117.148] (ovpn-117-148.ams2.redhat.com [10.36.117.148]) by smtp.corp.redhat.com (Postfix) with ESMTP id ECE0B5DA70; Wed, 11 Dec 2019 12:07:29 +0000 (UTC) Subject: Re: [RFC PATCH 2/4] mm/hotplug: Expose is_mem_section_removable() and offline_pages() To: lantianyu1986@gmail.com, kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com, sashal@kernel.org, akpm@linux-foundation.org, michael.h.kelley@microsoft.com Cc: Tianyu Lan , linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, vkuznets@redhat.com, eric.devolder@oracle.com References: <20191210154611.10958-1-Tianyu.Lan@microsoft.com> <20191210154611.10958-3-Tianyu.Lan@microsoft.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <20191210154611.10958-3-Tianyu.Lan@microsoft.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-MC-Unique: rAXCa65cO06Lllj2RyUgBg-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Sender: linux-hyperv-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org On 10.12.19 16:46, lantianyu1986@gmail.com wrote: > From: Tianyu Lan >=20 > Hyper-V driver adds memory hot remove function and will use > these interfaces in Hyper-V balloon driver which may be built > as a module. Expose these function. This patches misses a detailed description how these interfaces will be used. Also, you should CC people on the actual magic where it will be used. I found it via https://lkml.org/lkml/2019/12/10/767 If I am not wrong (un)lock_device_hotplug() is not exposed to kernel modules for a good reason - your patch seems to ignore that if I am not wrong. >=20 > Signed-off-by: Tianyu Lan > --- > mm/memory_hotplug.c | 2 ++ > 1 file changed, 2 insertions(+) >=20 > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c > index 07e5c67f48a8..4b358ebcc3d7 100644 > --- a/mm/memory_hotplug.c > +++ b/mm/memory_hotplug.c > @@ -1191,6 +1191,7 @@ bool is_mem_section_removable(unsigned long start_p= fn, unsigned long nr_pages) > =09/* All pageblocks in the memory block are likely to be hot-removable = */ > =09return true; > } > +EXPORT_SYMBOL_GPL(is_mem_section_removable); > =20 > /* > * Confirm all pages in a range [start, end) belong to the same zone. > @@ -1612,6 +1613,7 @@ int offline_pages(unsigned long start_pfn, unsigned= long nr_pages) > { > =09return __offline_pages(start_pfn, start_pfn + nr_pages); > } > +EXPORT_SYMBOL_GPL(offline_pages); > =20 > static int check_memblock_offlined_cb(struct memory_block *mem, void *ar= g) > { >=20 No, I don't think exposing the latter is desired. We already have one other in-tree user that I _really_ want to get rid of. Memory should be offlined in memory block granularity via the core only. Memory offlining can be triggered in a clean way via device_offline(&mem->dev). a) It conflicts with activity from user space. Especially, this "manual fixup" of the memory block state is just nasty. b) Locking issues: Memory offlining requires the device hotplug lock. This lock is not exposed and we don't want to expose it. c) There are still cases where offline_pages() will loop for all eternity and only signals can kick it out. E.g., have a look at how I with virtio-mem want to achieve that: https://lkml.org/lkml/2019/9/19/476 I think something like that would be *much* cleaner. What could be even better for your use case is doing it similarly to virtio-mem: 1. Try to alloc_contig_range() the memory block you want to remove. This will not loop forever but fail in a nice way early. See https://lkml.org/lkml/2019/9/19/467 2. Allow to offline that memory block by marking the memory PageOffline() and dropping the refcount. See https://lkml.org/lkml/2019/9/19/470, I will send a new RFC v4 soon that includes the suggestion from Michal. 3. Offline+remove the memory block using a clean interface. See https://lkml.org/lkml/2019/9/19/476 No looping forever, no races with user space, no messing with memory block states. NACK on exporting offline_pages(), but I am not a Maintainer, so ... :) --=20 Thanks, David / dhildenb