linux-kernel - Re: [PATCH v4 3/9] mm/lru: replace pgdat lru

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Message-ID: <20191119164448.GA396644@cmpxchg.org>
Date:   Tue, 19 Nov 2019 11:44:48 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Alex Shi <alex.shi@...ux.alibaba.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        tj@...nel.org, hughd@...gle.com, khlebnikov@...dex-team.ru,
        daniel.m.jordan@...cle.com, yang.shi@...ux.alibaba.com,
        shakeelb@...gle.com, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>,
        Chris Down <chris@...isdown.name>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, Qian Cai <cai@....pw>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        swkhack <swkhack@...il.com>,
        "Potyra, Stefan" <Stefan.Potyra@...ktrobit.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Colin Ian King <colin.king@...onical.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Peng Fan <peng.fan@....com>,
        Nikolay Borisov <nborisov@...e.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v4 3/9] mm/lru: replace pgdat lru_lock with lruvec lock

On Tue, Nov 19, 2019 at 07:57:04AM -0800, Matthew Wilcox wrote:
> On Tue, Nov 19, 2019 at 08:23:17PM +0800, Alex Shi wrote:
> > +static inline struct lruvec *lock_page_lruvec_irqsave(struct page *page,
> > +				struct pglist_data *pgdat, unsigned long *flags)
> > +{
> > +	struct lruvec *lruvec = mem_cgroup_page_lruvec(page, pgdat);
> > +
> > +	spin_lock_irqsave(&lruvec->lru_lock, *flags);
> > +
> > +	return lruvec;
> > +}
> 
> This should be a macro, not a function.  You basically can't do this;
> spin_lock_irqsave needs to write to a variable which can then be passed
> to spin_unlock_irqrestore().  What you're doing here will dereference the
> pointer in _this_ function, but won't propagate the modified value back to
> the caller.  I suppose you could do something like this ...

This works because spin_lock_irqsave and local_irq_save() are
macros. It boils down to '*flags = arch_local_irq_save()' in this
function, and therefor does the right thing.

We exploit that in quite a few places:

$ git grep 'spin_lock_irqsave(.*\*flags' | wc -l
39