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Thu, 27 Dec 2018 21:19:24 -0800 (PST) Date: Fri, 28 Dec 2018 14:19:20 +0900 From: Minchan Kim To: Sergey Senozhatsky Cc: Andrew Morton , LKML , joaodias@google.com, srnvs@google.com Subject: Re: [PATCH] zram: idle writeback fixes and cleanup Message-ID: <20181228051920.GA63763@google.com> References: <20181224033529.19450-1-minchan@kernel.org> <20181227022624.GA30091@jagdpanzerIV> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181227022624.GA30091@jagdpanzerIV> User-Agent: Mutt/1.10.1+60 (6df12dc1) (2018-08-07) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Sergey, On Thu, Dec 27, 2018 at 11:26:24AM +0900, Sergey Senozhatsky wrote: > On (12/24/18 12:35), Minchan Kim wrote: > [..] > > @@ -645,10 +680,13 @@ static ssize_t writeback_store(struct device *dev, > > bvec.bv_len = PAGE_SIZE; > > bvec.bv_offset = 0; > > > > - if (zram->stop_writeback) { > > + spin_lock(&zram->wb_limit_lock); > > + if (zram->wb_limit_enable && !zram->bd_wb_limit) { > > + spin_unlock(&zram->wb_limit_lock); > > ret = -EIO; > > break; > > } > > + spin_unlock(&zram->wb_limit_lock); > [..] > > @@ -732,11 +771,10 @@ static ssize_t writeback_store(struct device *dev, > > zram_set_element(zram, index, blk_idx); > > blk_idx = 0; > > atomic64_inc(&zram->stats.pages_stored); > > - if (atomic64_add_unless(&zram->stats.bd_wb_limit, > > - -1 << (PAGE_SHIFT - 12), 0)) { > > - if (atomic64_read(&zram->stats.bd_wb_limit) == 0) > > - zram->stop_writeback = true; > > - } > > + spin_lock(&zram->wb_limit_lock); > > + if (zram->wb_limit_enable && zram->bd_wb_limit > 0) > > + zram->bd_wb_limit -= 1UL << (PAGE_SHIFT - 12); > > + spin_unlock(&zram->wb_limit_lock); > > Do we really need ->wb_limit_lock spinlock? We kinda punch it twice > in this loop. If someone clears ->wb_limit_enable somewhere in between > then the worst thing to happen is that we will just write extra page > to the backing device; not a very big deal to me. Am I missing > something? Without the lock, bd_wb_limit store/read would be racy. CPU A CPU B if (zram->wb_limit_enable && zram->bd_wb_limit > 0) zram->bd_wb_limit = 0 zram->bd_wb_limit -= 1UL << (PAGE_SHIFT - 12) It makes limit feature void. > > -ss