linux-kernel - Re: Extending page pinning into fs/direct-io.c

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Date:   Thu, 25 May 2023 10:13:39 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Extending page pinning into fs/direct-io.c

On Thu, May 25, 2023 at 10:01 AM David Howells <dhowells@...hat.com> wrote:
>
> What do we gain from it?  Presumably since nothing is supposed to write to
> 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