linux-kernel - Re: [PATCH -mmotm 3/4] memcg: dirty pages accounting and limiting infrastructure

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Date:	Mon, 8 Mar 2010 11:37:11 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc:	Andrea Righi <arighi@...eler.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Trond Myklebust <trond.myklebust@....uio.no>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Greg Thelen <gthelen@...gle.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mmotm 3/4] memcg: dirty pages accounting and limiting
 infrastructure

On Mon, 8 Mar 2010 11:17:24 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:

> > But IIRC, clear_writeback is done under treelock.... No ?
> > 
> The place where NR_WRITEBACK is updated is out of tree_lock.
> 
>    1311 int test_clear_page_writeback(struct page *page)
>    1312 {
>    1313         struct address_space *mapping = page_mapping(page);
>    1314         int ret;
>    1315
>    1316         if (mapping) {
>    1317                 struct backing_dev_info *bdi = mapping->backing_dev_info;
>    1318                 unsigned long flags;
>    1319
>    1320                 spin_lock_irqsave(&mapping->tree_lock, flags);
>    1321                 ret = TestClearPageWriteback(page);
>    1322                 if (ret) {
>    1323                         radix_tree_tag_clear(&mapping->page_tree,
>    1324                                                 page_index(page),
>    1325                                                 PAGECACHE_TAG_WRITEBACK);
>    1326                         if (bdi_cap_account_writeback(bdi)) {
>    1327                                 __dec_bdi_stat(bdi, BDI_WRITEBACK);
>    1328                                 __bdi_writeout_inc(bdi);
>    1329                         }
>    1330                 }
>    1331                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
>    1332         } else {
>    1333                 ret = TestClearPageWriteback(page);
>    1334         }
>    1335         if (ret)
>    1336                 dec_zone_page_state(page, NR_WRITEBACK);
>    1337         return ret;
>    1338 }

We can move this up to under tree_lock. Considering memcg, all our target has "mapping".

If we newly account bounce-buffers (for NILFS, FUSE, etc..), which has no ->mapping,
we need much more complex new charge/uncharge theory.

But yes, adding new lock scheme seems complicated. (Sorry Andrea.)
My concerns is performance. We may need somehing new re-implementation of
locks/migrate/charge/uncharge.

Thanks,
-Kame



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