linux-kernel - Re: [PATCH 3/6] lib/bucket_locks: use kvmalloc

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Date:   Thu, 24 May 2018 14:37:36 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Graf <tgraf@...g.ch>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Manfred Spraul <manfred@...orfullife.com>,
        guillaume.knispel@...ersonicimagine.com,
        Linux API <linux-api@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 3/6] lib/bucket_locks: use kvmalloc_array()

On Thu, May 24, 2018 at 2:28 PM Davidlohr Bueso <dave@...olabs.net> wrote:

>                  if (gfpflags_allow_blocking(gfp))
> -                       tlocks = kvmalloc(size * sizeof(spinlock_t), gfp);
> +                       tlocks = kvmalloc_array(size, sizeof(spinlock_t),
gfp);
>                  else
>                          tlocks = kmalloc_array(size, sizeof(spinlock_t),
gfp);

Side note: how about we just move that "gfpflags_allow_blocking()" into
kvmalloc() instead, and make kvmalloc() generally usable?

Now we have that really odd situation where kvmalloc() takes gfp flags, but
to quote the comment:

  * Any use of gfp flags outside of GFP_KERNEL should be consulted with mm
people.

and the code:

         /*
          * vmalloc uses GFP_KERNEL for some internal allocations (e.g page
tables)
          * so the given set of flags has to be compatible.
          */
         WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);

which isn't really all that helpful. Do mm people really want to be
consulted about random uses?

Maybe we could just make the rule for kvmalloc() be to only fall back on
vmalloc for allocations that are

  - larger than page size

  - blocking and allow GFP_KERNEL (so basically that WARN_ON_ONCE() logic in
kvmalloc_node).

Hmm? Isn't that what everybody really *wants* kvmalloc() and friends to do?

              Linus