From mboxrd@z Thu Jan  1 00:00:00 1970
From: Vasily Averin <vvs-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
Subject: Re: [PATCH 2/2] ipc: use kmalloc for msg_queue and shmid_kernel
Date: Wed, 28 Apr 2021 08:15:10 +0300
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To: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Alexey Dobriyan <adobriyan-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>, Vladimir Davydov <vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Dmitry Safonov <0x7f454c46-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>

On 4/26/21 1:25 PM, Michal Hocko wrote:
> Using kvmalloc for sub page size objects is suboptimal because kmalloc
> can easily fallback into vmalloc under memory pressure and smaller
> objects would fragment memory. Therefore replace kvmalloc by a simple
> kmalloc.

I think you're wrong here:
kvmalloc can failback to vmalloc for size > PAGE_SIZE only

Please take look at mm/util.c::kvmalloc_node()

        if (size > PAGE_SIZE) {
                kmalloc_flags |= __GFP_NOWARN;

                if (!(kmalloc_flags & __GFP_RETRY_MAYFAIL))
                        kmalloc_flags |= __GFP_NORETRY;
        }

        ret = kmalloc_node(size, kmalloc_flags, node);

        /*
         * It doesn't really make sense to fallback to vmalloc for sub page
         * requests
         */
        if (ret || size <= PAGE_SIZE)
                return ret;

        return __vmalloc_node(size, 1, flags, node,
                        __builtin_return_address(0));

For small objects kvmalloc is not much different just from kmalloc,
so the patch is mostly cosmetic.