Re: [PATCH v5 02/16] mm: introduce kmem_cache_alloc_lru

From: Vlastimil Babka <vbabka@suse.cz>
To: Roman Gushchin <guro@fb.com>, Muchun Song <songmuchun@bytedance.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Shakeel Butt <shakeelb@google.com>,
	Yang Shi <shy828301@gmail.com>, Alex Shi <alexs@kernel.org>,
	Wei Yang <richard.weiyang@gmail.com>,
	Dave Chinner <david@fromorbit.com>,
	trond.myklebust@hammerspace.com, anna.schumaker@netapp.com,
	jaegeuk@kernel.org, chao@kernel.org,
	Kari Argillander <kari.argillander@gmail.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux Memory Management List <linux-mm@kvack.org>,
	linux-nfs@vger.kernel.org, Qi Zheng <zhengqi.arch@bytedance.com>,
	Xiongchun duan <duanxiongchun@bytedance.com>,
	Fam Zheng <fam.zheng@bytedance.com>,
	Muchun Song <smuchun@gmail.com>, Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>
Subject: Re: [PATCH v5 02/16] mm: introduce kmem_cache_alloc_lru
Date: Tue, 11 Jan 2022 16:41:29 +0100	[thread overview]
Message-ID: <a72a4e3a-3af9-7a36-4583-6181f3579cfb@suse.cz> (raw)
In-Reply-To: <Ydx/MFK72xrsXE0l@carbon.dhcp.thefacebook.com>

On 1/10/22 19:47, Roman Gushchin wrote:
> 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@fb.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.

I'm not sure how a flag would help as it seems we really need to pass a
specific list_lru pointer and work with that. I was thinking if there was
only one list_lru per class of object it could be part of struct kmem_cache,
but investigating kmem_cache_alloc_lru() callers I see lru parameters:

- &nfs4_xattr_cache_lru - this is fixed
- xas->xa_lru potentially not fixed, although the only caller of
xas_set_lru() passes &shadow_nodes so effectively fixed
- &sb->s_dentry_lru - dynamic, boo

> Vlastimil, what do you think?

Memcg code is already quite intertwined with slab code, for better or worse,
so I guess the extra lru parameter in a bunch of inline functions won't
change much. I don't immediately see a better solution.

> Thanks!
> 