linux-kernel - Re: [PATCH v6 12/16] mm: list_lru: replace linear array with xarray

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Message-ID: <CAMZfGtXiVDxO1hqiOChR2a_nwK5kUiWafEXML7=YfuPBy0zH5w@mail.gmail.com>
Date:   Fri, 1 Apr 2022 11:23:01 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Yang Shi <shy828301@...il.com>, Alex Shi <alexs@...nel.org>,
        Wei Yang <richard.weiyang@...il.com>,
        Dave Chinner <david@...morbit.com>,
        trond.myklebust@...merspace.com, anna.schumaker@...app.com,
        jaegeuk@...nel.org, chao@...nel.org,
        Kari Argillander <kari.argillander@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-nfs@...r.kernel.org, Qi Zheng <zhengqi.arch@...edance.com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        Fam Zheng <fam.zheng@...edance.com>,
        Muchun Song <smuchun@...il.com>
Subject: Re: [PATCH v6 12/16] mm: list_lru: replace linear array with xarray

On Thu, Mar 31, 2022 at 11:01 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Feb 28, 2022 at 08:21:22PM +0800, Muchun Song wrote:
> > @@ -586,27 +508,48 @@ int memcg_list_lru_alloc(struct mem_cgroup *memcg, struct list_lru *lru,
> >               }
> >       }
> >
> > +     xas_lock_irqsave(&xas, flags);
> >       while (i--) {
> > +             int index = READ_ONCE(table[i].memcg->kmemcg_id);
> >               struct list_lru_per_memcg *mlru = table[i].mlru;
> >
> > +             xas_set(&xas, index);
> > +retry:
> > +             if (unlikely(index < 0 || xas_error(&xas) || xas_load(&xas))) {
> >                       kfree(mlru);
> > +             } else {
> > +                     xas_store(&xas, mlru);
> > +                     if (xas_error(&xas) == -ENOMEM) {
> > +                             xas_unlock_irqrestore(&xas, flags);
> > +                             if (xas_nomem(&xas, gfp))
> > +                                     xas_set_err(&xas, 0);
> > +                             xas_lock_irqsave(&xas, flags);
> > +                             /*
> > +                              * The xas lock has been released, this memcg
> > +                              * can be reparented before us. So reload
> > +                              * memcg id. More details see the comments
> > +                              * in memcg_reparent_list_lrus().
> > +                              */
> > +                             index = READ_ONCE(table[i].memcg->kmemcg_id);
> > +                             if (index < 0)
> > +                                     xas_set_err(&xas, 0);
> > +                             else if (!xas_error(&xas) && index != xas.xa_index)
> > +                                     xas_set(&xas, index);
> > +                             goto retry;
> > +                     }
> > +             }
> >       }
> > +     /* xas_nomem() is used to free memory instead of memory allocation. */
> > +     if (xas.xa_alloc)
> > +             xas_nomem(&xas, gfp);
> > +     xas_unlock_irqrestore(&xas, flags);
> >       kfree(table);
> >
> > +     return xas_error(&xas);
> >  }
>
> This is really unidiomatic.  And so much more complicated than it needs
> to be.
>
>         while (i--) {
>                 do {
>                         int index = READ_ONCE(table[i].memcg->kmemcg_id);
>                         struct list_lru_per_memcg *mlru = table[i].mlru;
>
>                         xas_set(&xas, index);
>                         xas_lock_irqsave(&xas, flags);
>                         if (index < 0 || xas_load(&xas))
>                                 xas_set_err(&xas, -EBUSY);
>                         xas_store(&xas, mlru);
>                         if (!xas_error(&xas))
>                                 break;
>                         xas_unlock_irqrestore(&xas, flags);
>                 } while (xas_nomem(&xas, gfp));
>
>                 if (xas_error(&xas))
>                         kfree(mlru);
>         }
>
>         kfree(table);
>         return xas_error(&xas);
>
> (you might want to mask out the EBUSY error here)
>

I'm not familiar with xarray APIs.  I'll dig deeper to see if this code works
properly.  Thanks for your suggestions.