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Message-Id: <20100308111724.3e48aee3.nishimura@mxp.nes.nec.co.jp>
Date: Mon, 8 Mar 2010 11:17:24 +0900
From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
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,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH -mmotm 3/4] memcg: dirty pages accounting and limiting
infrastructure
On Mon, 8 Mar 2010 10:56:41 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> On Mon, 8 Mar 2010 10:44:47 +0900
> Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
>
> > > +/*
> > > + * mem_cgroup_update_page_stat_locked() - update memcg file cache's accounting
> > > + * @page: the page involved in a file cache operation.
> > > + * @idx: the particular file cache statistic.
> > > + * @charge: true to increment, false to decrement the statistic specified
> > > + * by @idx.
> > > + *
> > > + * Update memory cgroup file cache's accounting from a locked context.
> > > + *
> > > + * NOTE: must be called with mapping->tree_lock held.
> > > + */
> > > +void mem_cgroup_update_page_stat_locked(struct page *page,
> > > + enum mem_cgroup_write_page_stat_item idx, bool charge)
> > > +{
> > > + struct address_space *mapping = page_mapping(page);
> > > + struct page_cgroup *pc;
> > > +
> > > + if (mem_cgroup_disabled())
> > > + return;
> > > + WARN_ON_ONCE(!irqs_disabled());
> > > + WARN_ON_ONCE(mapping && !spin_is_locked(&mapping->tree_lock));
> > > +
> > I think this is a wrong place to insert assertion.
> > The problem about page cgroup lock is that it can be interrupted in current implementation.
> > So,
> >
> > a) it must not be aquired under another lock which can be aquired in interrupt context,
> > such as mapping->tree_lock, to avoid:
> >
> > context1 context2
> > lock_page_cgroup(pcA)
> > spin_lock_irq(&tree_lock)
> > lock_page_cgroup(pcA) <interrupted>
> > =>fail spin_lock_irqsave(&tree_lock)
> > =>fail
> >
> > b) it must not be aquired in interrupt context to avoid:
> >
> > lock_page_cgroup(pcA)
> > <interrupted>
> > lock_page_cgroup(pcA)
> > =>fail
> >
> > I think something like this would be better:
> >
> > @@ -83,8 +83,14 @@ static inline enum zone_type page_cgroup_zid(struct page_cgroup *pc)
> > return page_zonenum(pc->page);
> > }
> >
> > +#include <linux/irqflags.h>
> > +#include <linux/hardirq.h>
> > static inline void lock_page_cgroup(struct page_cgroup *pc)
> > {
> > +#ifdef CONFIG_DEBUG_VM
> > + WARN_ON_ONCE(irqs_disabled());
> > + WARN_ON_ONCE(in_interrupt());
> > +#endif
> > bit_spin_lock(PCG_LOCK, &pc->flags);
> > }
> >
> > > + pc = lookup_page_cgroup(page);
> > > + if (unlikely(!pc) || !PageCgroupUsed(pc))
> > > + return;
> > > + mem_cgroup_update_page_stat(pc, idx, charge);
> > > +}
> > > +EXPORT_SYMBOL_GPL(mem_cgroup_update_page_stat_locked);
> > > +
> > > +/*
> > > + * mem_cgroup_update_page_stat_unlocked() - update memcg file cache's accounting
> > > + * @page: the page involved in a file cache operation.
> > > + * @idx: the particular file cache statistic.
> > > + * @charge: true to increment, false to decrement the statistic specified
> > > + * by @idx.
> > > + *
> > > + * Update memory cgroup file cache's accounting from an unlocked context.
> > > + */
> > > +void mem_cgroup_update_page_stat_unlocked(struct page *page,
> > > + enum mem_cgroup_write_page_stat_item idx, bool charge)
> > > +{
> > > + struct page_cgroup *pc;
> > > +
> > > + if (mem_cgroup_disabled())
> > > + return;
> > > + pc = lookup_page_cgroup(page);
> > > + if (unlikely(!pc) || !PageCgroupUsed(pc))
> > > + return;
> > > + lock_page_cgroup(pc);
> > > + mem_cgroup_update_page_stat(pc, idx, charge);
> > > unlock_page_cgroup(pc);
> > > }
> > > +EXPORT_SYMBOL_GPL(mem_cgroup_update_page_stat_unlocked);
> > >
> > IIUC, test_clear_page_writeback(at least) can be called under interrupt context.
> > This means lock_page_cgroup() is called under interrupt context, that is,
> > the case b) above can happen.
> > hmm... I don't have any good idea for now except disabling irq around page cgroup lock
> > to avoid all of these mess things.
> >
>
> Hmm...simply IRQ-off for all updates ?
I think so in current code.
But after these changes, we must use local_irq_save()/restore()
instead of local_irq_disable()/enable() in mem_cgroup_update_page_stat().
> 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 }
1339
Thanks,
Daisuke Nishimura.
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