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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 8 Mar 2010 11:17:24 +0900
Daisuke Nishimura <nishimura@mxp.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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