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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 25 May 2023 10:13:39 -0700
Message-ID: <CAHk-=wguy6bcoUznDCU3mr-wbi-8NigkTw5GvwiF8R76J=vGUQ@mail.gmail.com>
Subject: Re: Extending page pinning into fs/direct-io.c
To:     David Howells <dhowells@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Hillf Danton <hdanton@sina.com>,
        Christian Brauner <brauner@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Thu, May 25, 2023 at 10:01=E2=80=AFAM David Howells <dhowells@redhat.com=
> wrote:
>
> What do we gain from it?  Presumably since nothing is supposed to write t=
o
> that page, it can be shared in all the caches.

I don't remember the details, but they went something like "broken
purely virtually indexed cache avoids physical aliases by cacheline
exclusion at fill time".

Which then meant that if you walk a zero mapping, you'll invalidate
the caches of the previous page when you walk the next one.  Causing
horrendously bad performance.

Unless it's colored.

Something like that. I probably got all the details wrong.

                Linus