linux-kernel - Re: [PATCH RFC 14/20] mempool: introduce mempool_use_prealloc

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Date:   Thu, 23 Nov 2023 19:06:16 +0100
From:   Andrey Konovalov <andreyknvl@...il.com>
To:     Marco Elver <elver@...gle.com>
Cc:     andrey.konovalov@...ux.dev,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        kasan-dev@...glegroups.com, Evgenii Stepanov <eugenis@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH RFC 14/20] mempool: introduce mempool_use_prealloc_only

On Wed, Nov 22, 2023 at 6:21 PM Marco Elver <elver@...gle.com> wrote:
>
> On Mon, Nov 06, 2023 at 09:10PM +0100, andrey.konovalov@...ux.dev wrote:
> > From: Andrey Konovalov <andreyknvl@...gle.com>
> >
> > Introduce a new mempool_use_prealloc_only API that tells the mempool to
> > only use the elements preallocated during the mempool's creation and to
> > not attempt allocating new ones.
> >
> > This API is required to test the KASAN poisoning/unpoisoning functinality
> > in KASAN tests, but it might be also useful on its own.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> > ---
> >  include/linux/mempool.h |  2 ++
> >  mm/mempool.c            | 27 ++++++++++++++++++++++++---
> >  2 files changed, 26 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/mempool.h b/include/linux/mempool.h
> > index 4aae6c06c5f2..822adf1e7567 100644
> > --- a/include/linux/mempool.h
> > +++ b/include/linux/mempool.h
> > @@ -18,6 +18,7 @@ typedef struct mempool_s {
> >       int min_nr;             /* nr of elements at *elements */
> >       int curr_nr;            /* Current nr of elements at *elements */
> >       void **elements;
> > +     bool use_prealloc_only; /* Use only preallocated elements */
>
> This increases the struct size from 56 to 64 bytes (64 bit arch).
> mempool_t is embedded in lots of other larger structs, and this may
> result in some unwanted bloat.
>
> Is there a way to achieve the same thing without adding a new bool to
> the mempool struct?

We could split out the part of mempool_alloc that uses preallocated
elements without what waiting part and expose it in another API
function named something like mempool_alloc_preallocated. Would that
be better?