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[91.12.100.159]) by smtp.gmail.com with ESMTPSA id d2sm11770212wrs.10.2021.05.03.03.13.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 May 2021 03:13:46 -0700 (PDT) To: Mike Rapoport Cc: linux-kernel@vger.kernel.org, Andrew Morton , "Michael S. Tsirkin" , Jason Wang , Alexey Dobriyan , "Matthew Wilcox (Oracle)" , Oscar Salvador , Michal Hocko , Roman Gushchin , Alex Shi , Steven Price , Mike Kravetz , Aili Yao , Jiri Bohac , "K. Y. Srinivasan" , Haiyang Zhang , Stephen Hemminger , Wei Liu , Naoya Horiguchi , linux-hyperv@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org References: <20210429122519.15183-1-david@redhat.com> <20210429122519.15183-8-david@redhat.com> <5a5a7552-4f0a-75bc-582f-73d24afcf57b@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v1 7/7] fs/proc/kcore: use page_offline_(freeze|unfreeze) Message-ID: <2f66cbfc-aa29-b3ef-4c6a-0da8b29b56f6@redhat.com> Date: Mon, 3 May 2021 12:13:45 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=BevCE+CO; spf=none (imf06.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: B45E3C0007C6 X-Stat-Signature: 4c3hpmqkfegsjnxfccfrigw1guoohite Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf06; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=170.10.133.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1620036832-560038 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 03.05.21 11:28, Mike Rapoport wrote: > On Mon, May 03, 2021 at 10:28:36AM +0200, David Hildenbrand wrote: >> On 02.05.21 08:34, Mike Rapoport wrote: >>> On Thu, Apr 29, 2021 at 02:25:19PM +0200, David Hildenbrand wrote: >>>> Let's properly synchronize with drivers that set PageOffline(). Unfr= eeze >>>> every now and then, so drivers that want to set PageOffline() can ma= ke >>>> progress. >>>> >>>> Signed-off-by: David Hildenbrand >>>> --- >>>> fs/proc/kcore.c | 15 +++++++++++++++ >>>> 1 file changed, 15 insertions(+) >>>> >>>> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c >>>> index 92ff1e4436cb..3d7531f47389 100644 >>>> --- a/fs/proc/kcore.c >>>> +++ b/fs/proc/kcore.c >>>> @@ -311,6 +311,7 @@ static void append_kcore_note(char *notes, size_= t *i, const char *name, >>>> static ssize_t >>>> read_kcore(struct file *file, char __user *buffer, size_t buflen,= loff_t *fpos) >>>> { >>>> + size_t page_offline_frozen =3D 0; >>>> char *buf =3D file->private_data; >>>> size_t phdrs_offset, notes_offset, data_offset; >>>> size_t phdrs_len, notes_len; >>>> @@ -509,6 +510,18 @@ read_kcore(struct file *file, char __user *buff= er, size_t buflen, loff_t *fpos) >>>> pfn =3D __pa(start) >> PAGE_SHIFT; >>>> page =3D pfn_to_online_page(pfn); >>> >>> Can't this race with page offlining for the first time we get here? >> >> >> To clarify, we have three types of offline pages in the kernel ... >> >> a) Pages part of an offline memory section; the memap is stale and not >> trustworthy. pfn_to_online_page() checks that. We *can* protect agains= t >> memory offlining using get_online_mems()/put_online_mems(), but usuall= y >> avoid doing so as the race window is very small (and a problem all ove= r the >> kernel we basically never hit) and locking is rather expensive. In the >> future, we might switch to rcu to handle that more efficiently and avo= iding >> these possible races. >> >> b) PageOffline(): logically offline pages contained in an online memor= y >> section with a sane memmap. virtio-mem calls these pages "fake offline= "; >> something like a "temporary" memory hole. The new mechanism I propose = will >> be used to handle synchronization as races can be more severe, e.g., w= hen >> reading actual page content here. >> >> c) Soft offline pages: hwpoisoned pages that are not actually harmful = yet, >> but could become harmful in the future. So we better try to remove the= page >> from the page allcoator and try to migrate away existing users. >> >> >> So page_offline_* handle "b) PageOffline()" only. There is a tiny race >> between pfn_to_online_page(pfn) and looking at the memmap as we have i= n many >> cases already throughout the kernel, to be tackled in the future. >=20 > Right, but here you anyway add locking, so why exclude the first iterat= ion? What we're protecting is PageOffline() below. If I didn't mess up, we=20 should always be calling page_offline_freeze() before calling=20 PageOffline(). Or am I missing something? >=20 > BTW, did you consider something like Yes, I played with something like that. We'd have to handle the first=20 page_offline_freeze() freeze differently, though, and that's where=20 things got a bit ugly in my attempts. >=20 > if (page_offline_frozen++ % MAX_ORDER_NR_PAGES =3D=3D 0) { > page_offline_unfreeze(); > cond_resched(); > page_offline_freeze(); > } >=20 > We don't seem to care about page_offline_frozen overflows here, do we? No, the buffer size is also size_t and gets incremented on a per-byte=20 basis. The variant I have right now looked the cleanest to me. Happy to=20 hear simpler alternatives. --=20 Thanks, David / dhildenb