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From:   Alexander Duyck <alexander.duyck@gmail.com>
Date:   Fri, 16 Jun 2023 09:06:19 -0700
Message-ID: <CAKgT0Udfud_wVKLiZbi4Okuzor7OKzRi_PwvvCwnsiOdw7ir7A@mail.gmail.com>
Subject: Re: [PATCH net-next 03/12] mm: Make the page_frag_cache allocator
 alignment param a pow-of-2
To:     David Howells <dhowells@redhat.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        David Ahern <dsahern@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Jeroen de Borst <jeroendb@google.com>,
        Catherine Sullivan <csully@google.com>,
        Shailend Chand <shailend@google.com>,
        Felix Fietkau <nbd@nbd.name>, John Crispin <john@phrozen.org>,
        Sean Wang <sean.wang@mediatek.com>,
        Mark Lee <Mark-MC.Lee@mediatek.com>,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
        Christoph Hellwig <hch@lst.de>,
        Sagi Grimberg <sagi@grimberg.me>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, linux-nvme@lists.infradead.org
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On Fri, Jun 16, 2023 at 8:28=E2=80=AFAM David Howells <dhowells@redhat.com>=
 wrote:
>
> Alexander H Duyck <alexander.duyck@gmail.com> wrote:
>
> > The requirement should only be cache alignment, not power of 2
> > alignment.
>
> Sure, but, upstream, page_frag_alloc_align() allows the specification of =
an
> alignment and applies that alignment by:
>
>         offset &=3D align_mask;
>
> which doesn't really make sense unless the alignment boils down to being =
a
> power of two.  Now, it might make sense to kill off the align_mask parame=
ter
> and just go with SMP_CACHE_BYTES (which will be a power of two).
>
> Note, though, that most users seem to use an align_mask of ~0u which feel=
s a
> bit dodgy (it's not an unsigned long), but is probably fine.

Yeah, now that I think about it this should be fine. I must have been
responding to this without enough sleep. When I originally read it I
saw it as limiting it to a power of 2 allocation, not limiting the
alignment mask to a power of 2.