linux-kernel - Re: [RFC] Re: Costless huge virtual memory? /dev/same, /dev/null?

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Message-ID: <20170413170456.77abmvqs6b6b3hfx@var.youpi.perso.aquilenet.fr>
Date:   Thu, 13 Apr 2017 19:04:56 +0200
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC] Re: Costless huge virtual memory? /dev/same, /dev/null?

Matthew Wilcox, on jeu. 13 avril 2017 09:34:11 -0700, wrote:
> On Thu, Apr 13, 2017 at 06:29:46PM +0200, Samuel Thibault wrote:
> > (Ideally we'd be able to take the MAP_HUGETLB mmap flag into account to
> > map a single huge page repeatedly, even lowering the populating cost,
> > but AIUI of the current hugepage support it would be far from easy)
> 
> You could implement ->map_pages instead of (or as well as) ->fault, which
> would lower your costs if that is a concern.

Yes it is a concern.  I was a bit afraid that implementing map_pages
would be frowned upon, and getting in first a simple yet already useful
/dev/garbage would work better :)

I'm fine with doing either.

> I think the eventual intent is that map_pages would be able to
> populate a PMD or even a PUD.

Yes.  Allocating such a big page can be a problem, though.

Samuel