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Mon, 20 Jun 2022 02:23:36 -0700 (PDT) Date: Mon, 20 Jun 2022 17:23:31 +0800 From: Muchun Song To: Miaohe Lin Cc: "Huang, Ying" , akpm@linux-foundation.org, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 2/3] mm/swapfile: fix possible data races of inuse_pages Message-ID: References: <20220608144031.829-1-linmiaohe@huawei.com> <20220608144031.829-3-linmiaohe@huawei.com> <87edzjrcq8.fsf@yhuang6-desk2.ccr.corp.intel.com> <13414d6a-9e72-fb6c-f0a8-8b83ba0455de@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <13414d6a-9e72-fb6c-f0a8-8b83ba0455de@huawei.com> ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=bytedance-com.20210112.gappssmtp.com header.s=20210112 header.b=2ZksTczO; dmarc=pass (policy=none) header.from=bytedance.com; spf=pass (imf24.hostedemail.com: domain of songmuchun@bytedance.com designates 209.85.214.174 as permitted sender) smtp.mailfrom=songmuchun@bytedance.com ARC-Message-Signature: i=1; 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dmarc=pass (policy=none) header.from=bytedance.com; spf=pass (imf24.hostedemail.com: domain of songmuchun@bytedance.com designates 209.85.214.174 as permitted sender) smtp.mailfrom=songmuchun@bytedance.com X-Rspamd-Server: rspam10 X-HE-Tag: 1655717018-642843 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 Mon, Jun 20, 2022 at 05:04:50PM +0800, Miaohe Lin wrote: > On 2022/6/20 15:54, Huang, Ying wrote: > > Miaohe Lin writes: > > > >> si->inuse_pages could still be accessed concurrently now. The plain reads > >> outside si->lock critical section, i.e. swap_show and si_swapinfo, which > >> results in data races. But these should be ok because they're just used > >> for showing swap info. > >> > >> Signed-off-by: Miaohe Lin > >> Reviewed-by: David Hildenbrand > >> --- > >> mm/swapfile.c | 4 ++-- > >> 1 file changed, 2 insertions(+), 2 deletions(-) > >> > >> diff --git a/mm/swapfile.c b/mm/swapfile.c > >> index d2bead7b8b70..3fa26f6971e9 100644 > >> --- a/mm/swapfile.c > >> +++ b/mm/swapfile.c > >> @@ -2646,7 +2646,7 @@ static int swap_show(struct seq_file *swap, void *v) > >> } > >> > >> bytes = si->pages << (PAGE_SHIFT - 10); > >> - inuse = si->inuse_pages << (PAGE_SHIFT - 10); > >> + inuse = READ_ONCE(si->inuse_pages) << (PAGE_SHIFT - 10); > >> > >> file = si->swap_file; > >> len = seq_file_path(swap, file, " \t\n\\"); > >> @@ -3265,7 +3265,7 @@ void si_swapinfo(struct sysinfo *val) > >> struct swap_info_struct *si = swap_info[type]; > >> > >> if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK)) > >> - nr_to_be_unused += si->inuse_pages; > >> + nr_to_be_unused += READ_ONCE(si->inuse_pages); > >> } > >> val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused; > >> val->totalswap = total_swap_pages + nr_to_be_unused; > > > > READ_ONCE() should be paired with WRITE_ONCE(). So, change the writer > > side too? > > READ_ONCE() is used to fix the complaint of concurrent accessing to si->inuse_pages from KCSAN here. > The similar commit is 218209487c3d ("mm/swapfile: fix data races in try_to_unuse()"). IMHO, it's fine I think the fix 218209487c3d is incomplete. The write side in swap_range_free() should also be fixed. Otherwise, IIUC, it cannot stop KCSAN complaining. > to see a not-uptodate value of si->inuse_pages because it's just used for showing swap info. So > WRITE_ONCE() is not obligatory. Or am I miss something? > > > > > Best Regards, > > Huang, Ying > > Thanks! > > > . > > > >