linux-kernel - Re: [PATCH v5 02/16] mm: introduce kmem_cache_alloc

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Message-ID: <Ydx/MFK72xrsXE0l@carbon.dhcp.thefacebook.com>
Date:   Mon, 10 Jan 2022 10:47:12 -0800
From:   Roman Gushchin <guro@...com>
To:     Muchun Song <songmuchun@...edance.com>,
        Vlastimil Babka <vbabka@...e.cz>
CC:     Matthew Wilcox <willy@...radead.org>,
        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>,
        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>,
        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>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v5 02/16] mm: introduce kmem_cache_alloc_lru

On Sun, Jan 09, 2022 at 02:21:22PM +0800, Muchun Song wrote:
> On Fri, Jan 7, 2022 at 11:05 AM Roman Gushchin <guro@...com> wrote:
> >
> [...]
> > >  /*
> > >   * struct kmem_cache related prototypes
> > > @@ -425,6 +426,8 @@ static __always_inline unsigned int __kmalloc_index(size_t size,
> > >
> > >  void *__kmalloc(size_t size, gfp_t flags) __assume_kmalloc_alignment __alloc_size(1);
> > >  void *kmem_cache_alloc(struct kmem_cache *s, gfp_t flags) __assume_slab_alignment __malloc;
> > > +void *kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
> > > +                        gfp_t gfpflags) __assume_slab_alignment __malloc;
> >
> > I'm not a big fan of this patch: I don't see why preparing the lru
> > infrastructure has to be integrated that deep into the slab code.
> >
> > Why can't kmem_cache_alloc_lru() be a simple wrapper like (pseudo-code):
> >   void *kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
> >                            gfp_t gfpflags) {
> >         if (necessarily)
> >            prepare_lru_infra();
> >         return kmem_cache_alloc();
> >   }
> 
> Hi Roman,
> 
> Actually, it can. But there is going to be some redundant code similar
> like memcg_slab_pre_alloc_hook() does to detect the necessity of
> prepare_lru_infra() in the new scheme of kmem_cache_alloc_lru().
> I just want to reduce the redundant overhead.

Is this about getting a memcg pointer?
I doubt it's a good reason to make changes all over the slab code.
Another option to consider adding a new gfp flag.

Vlastimil, what do you think?

Thanks!