Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936411Ab3DIBCj (ORCPT ); Mon, 8 Apr 2013 21:02:39 -0400 Received: from LGEMRELSE7Q.lge.com ([156.147.1.151]:52647 "EHLO LGEMRELSE7Q.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763340Ab3DIBCi (ORCPT ); Mon, 8 Apr 2013 21:02:38 -0400 X-AuditID: 9c930197-b7b50ae00000018c-10-516368ab7ae0 Date: Tue, 9 Apr 2013 10:02:31 +0900 From: Minchan Kim To: Andrew Morton Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Hugh Dickins , Seth Jennings , Nitin Gupta , Konrad Rzeszutek Wilk , Shaohua Li , Dan Magenheimer Subject: Re: [PATCH] mm: remove compressed copy from zram in-memory Message-ID: <20130409010231.GA3467@blaptop> References: <1365400862-9041-1-git-send-email-minchan@kernel.org> <20130408141710.1a1f76a0054bba49a42c76ca@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130408141710.1a1f76a0054bba49a42c76ca@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-Brightmail-Tracker: AAAAAA== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4093 Lines: 118 Hi Andrew, On Mon, Apr 08, 2013 at 02:17:10PM -0700, Andrew Morton wrote: > On Mon, 8 Apr 2013 15:01:02 +0900 Minchan Kim wrote: > > > Swap subsystem does lazy swap slot free with expecting the page > > would be swapped out again so we can avoid unnecessary write. > > Is that correct? How can it save a write? Correct. The add_to_swap makes the page dirty and we must pageout only if the page is dirty. If a anon page is already charged into swapcache, we skip writeout the page in shrink_page_list, then just remove the page from swapcache and free it by __remove_mapping. I did received same question multiple time so it would be good idea to write down it in vmscan.c somewhere. > > > But the problem in in-memory swap(ex, zram) is that it consumes > > memory space until vm_swap_full(ie, used half of all of swap device) > > condition meet. It could be bad if we use multiple swap device, > > small in-memory swap and big storage swap or in-memory swap alone. > > > > This patch makes swap subsystem free swap slot as soon as swap-read > > is completed and make the swapcache page dirty so the page should > > be written out the swap device to reclaim it. > > It means we never lose it. > > >From my reading of the patch, that isn't how it works? It changed > end_swap_bio_read() to call zram_slot_free_notify(), which appears to > free the underlying compressed page. I have a feeling I'm hopelessly > confused. You understand right totally. Selecting swap slot in my description was totally miss. Need to rewrite the description. > > > --- a/mm/page_io.c > > +++ b/mm/page_io.c > > @@ -20,6 +20,7 @@ > > #include > > #include > > #include > > +#include > > #include > > > > static struct bio *get_swap_bio(gfp_t gfp_flags, > > @@ -81,8 +82,30 @@ void end_swap_bio_read(struct bio *bio, int err) > > iminor(bio->bi_bdev->bd_inode), > > (unsigned long long)bio->bi_sector); > > } else { > > + /* > > + * There is no reason to keep both uncompressed data and > > + * compressed data in memory. > > + */ > > + struct swap_info_struct *sis; > > + > > SetPageUptodate(page); > > + sis = page_swap_info(page); > > + if (sis->flags & SWP_BLKDEV) { > > + struct gendisk *disk = sis->bdev->bd_disk; > > + if (disk->fops->swap_slot_free_notify) { > > + swp_entry_t entry; > > + unsigned long offset; > > + > > + entry.val = page_private(page); > > + offset = swp_offset(entry); > > + > > + SetPageDirty(page); > > + disk->fops->swap_slot_free_notify(sis->bdev, > > + offset); > > + } > > + } > > } > > + > > unlock_page(page); > > bio_put(bio); > > The new code is wasted space if CONFIG_BLOCK=n, yes? CONFIG_SWAP is already dependent on CONFIG_BLOCK. > > Also, what's up with the SWP_BLKDEV test? zram doesn't support > SWP_FILE? Why on earth not? > > Putting swap_slot_free_notify() into block_device_operations seems > rather wrong. It precludes zram-over-swapfiles for all time and means > that other subsystems cannot get notifications for swap slot freeing > for swapfile-backed swap. Zram is just pseudo-block device so anyone can format it with any FSes and swapon a file. In such case, he can't get a benefit from swap_slot_free_notify. But I think it's not a severe problem because there is no reason to use a file-swap on zram. If anyone want to use it, I'd like to know the reason. If it's reasonable, we have to rethink a wheel and it's another story, IMHO. > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/