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

[Linus Torvalds]

On Thu, May 25, 2023 at 10:01 AM David Howells <dhowells@xxxxxxxxxx> 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