linux-kernel - Re: [PATCH net-next 03/12] mm: Make the page_frag_cache allocator alignment param a pow-of-2

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Date:   Fri, 16 Jun 2023 09:06:19 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Jeroen de Borst <jeroendb@...gle.com>,
        Catherine Sullivan <csully@...gle.com>,
        Shailend Chand <shailend@...gle.com>,
        Felix Fietkau <nbd@....name>, John Crispin <john@...ozen.org>,
        Sean Wang <sean.wang@...iatek.com>,
        Mark Lee <Mark-MC.Lee@...iatek.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH net-next 03/12] mm: Make the page_frag_cache allocator
 alignment param a pow-of-2

On Fri, Jun 16, 2023 at 8:28 AM David Howells <dhowells@...hat.com> wrote:
>
> Alexander H Duyck <alexander.duyck@...il.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 &= 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 parameter
> 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 feels 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.