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

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Date:   Mon, 25 Nov 2019 09:27:25 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tejun Heo <tj@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Matthew Wilcox <willy@...radead.org>,
        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 Mon, Nov 25, 2019 at 1:26 AM Alex Shi <alex.shi@...ux.alibaba.com> wrote:
>
>
> >
> > But that leaves me with one more worry: compaction. We locked out
> > charge moving now, so between that and knowing that the page is alive,
> > we have page->mem_cgroup stable. But compaction doesn't know whether
> > the page is alive - it comes from a pfn and finds out using PageLRU.
> >
> > In the current code, pgdat->lru_lock remains the same before and after
> > the page is charged to a cgroup, so once compaction has that locked
> > and it observes PageLRU, it can go ahead and isolate the page.
> >
> > But lruvec->lru_lock changes during charging, and then compaction may
> > hold the wrong lock during isolation:
> >
> > compaction:                           generic_file_buffered_read:
> >
> >                                       page_cache_alloc()
> >
> > !PageBuddy()
> >
> > lock_page_lruvec(page)
> >   lruvec = mem_cgroup_page_lruvec()
> >   spin_lock(&lruvec->lru_lock)
> >   if lruvec != mem_cgroup_page_lruvec()
> >     goto again
> >
> >                                       add_to_page_cache_lru()
> >                                         mem_cgroup_commit_charge()
> >                                           page->mem_cgroup = foo
> >                                         lru_cache_add()
> >                                           __pagevec_lru_add()
> >                                             SetPageLRU()
> >
> > if PageLRU(page):
> >   __isolate_lru_page()
> >
> > I don't see what prevents the lruvec from changing under compaction,
> > neither in your patches nor in Hugh's. Maybe I'm missing something?
> >
>
> Hi Johannes,
>
> It looks my patch do the lruvec recheck/relock after PageLRU in compaction.c.
> It should be fine for your question. So I will try more testing after all changes.

Actually no, unless PageLRU check and taking lruvec lock are atomic,
the race mentioned by Johannes still exist.

Shakeel