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[46.53.177.92]) by smtp.gmail.com with ESMTPSA id o92-v6sm892261edb.85.2018.07.26.09.26.39 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 26 Jul 2018 09:26:39 -0700 (PDT) Date: Thu, 26 Jul 2018 19:26:37 +0300 From: Alexey Dobriyan To: Vlastimil Babka Cc: Andrew Morton , Daniel Colascione , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org Subject: Re: [PATCH 4/4] mm: proc/pid/smaps_rollup: convert to single value seq_file Message-ID: <20180726162637.GB25227@avx2> References: <20180723111933.15443-1-vbabka@suse.cz> <20180723111933.15443-5-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 25, 2018 at 08:53:53AM +0200, Vlastimil Babka wrote: > I moved the reply to this thread since the "added to -mm tree" > notification Alexey replied to in <20180724182908.GD27053@avx2> has > reduced CC list and is not linked to the patch postings. > > On 07/24/2018 08:29 PM, Alexey Dobriyan wrote: > > On Mon, Jul 23, 2018 at 04:55:48PM -0700, akpm@linux-foundation.org wrote: > >> The patch titled > >> Subject: mm: /proc/pid/smaps_rollup: convert to single value seq_file > >> has been added to the -mm tree. Its filename is > >> mm-proc-pid-smaps_rollup-convert-to-single-value-seq_file.patch > > > >> Subject: mm: /proc/pid/smaps_rollup: convert to single value seq_file > >> > >> The /proc/pid/smaps_rollup file is currently implemented via the > >> m_start/m_next/m_stop seq_file iterators shared with the other maps files, > >> that iterate over vma's. However, the rollup file doesn't print anything > >> for each vma, only accumulate the stats. > > > > What I don't understand why keep seq_ops then and not do all the work in > > ->show hook. Currently /proc/*/smaps_rollup is at ~500 bytes so with > > minimum 1 page seq buffer, no buffer resizing is possible. > > Hmm IIUC seq_file also provides the buffer and handles feeding the data > from there to the user process, which might have called read() with a smaller > buffer than that. So I would rather not avoid the seq_file infrastructure. > Or you're saying it could be converted to single_open()? Maybe, with more work. Prefereably yes. There are 2 ways to using seq_file: * introduce seq_operations and iterate over objects printing them one by one, * use single_open and 1 ->show hook and do all the work of collecting data there and print once. /proc/*/smaps_rollup is suited for variant 2 because variant 1 is designed for printing arbitrary amount of data. > >> +static int show_smaps_rollup(struct seq_file *m, void *v) > >> +{ > >> + struct proc_maps_private *priv = m->private; > >> + struct mem_size_stats *mss = priv->rollup; > >> + struct vm_area_struct *vma; > >> + > >> + /* > >> + * We might be called multiple times when e.g. the seq buffer > >> + * overflows. Gather the stats only once. > > > > It doesn't! > > Because the buffer is 1 page and the data is ~500 bytes as you said above? > Agreed, but I wouldn't want to depend on data not growing in the future or > the initial buffer not getting smaller. I could extend the comment that this > is theoretical for now? Given the rate of growth I wouldn't be concerned. > >> + if (!mss->finished) { > >> + for (vma = priv->mm->mmap; vma; vma = vma->vm_next) { > >> + smap_gather_stats(vma, mss); > >> + mss->last_vma_end = vma->vm_end; > >> } > >> - last_vma = !m_next_vma(priv, vma); > >> - } else { > >> - rollup_mode = false; > >> - memset(&mss_stack, 0, sizeof(mss_stack)); > >> - mss = &mss_stack;