linux-kernel - Re: [EXTERNAL] Re: [PATCH v3] mm/thp: fix "mm: thp: kill __transhuge_page

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Message-ID: <ZOijSwCa9NFD6DZI@casper.infradead.org>
Date:   Fri, 25 Aug 2023 13:49:15 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Yang Shi <shy828301@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Saurabh Sengar <ssengar@...ux.microsoft.com>
Subject: Re: [EXTERNAL] Re: [PATCH v3] mm/thp: fix "mm: thp: kill
 __transhuge_page_enabled()"

On Fri, Aug 25, 2023 at 09:59:23AM +0200, David Hildenbrand wrote:
> Especially, we do have bigger ->huge_fault changes coming up:
> 
> https://lkml.kernel.org/r/20230818202335.2739663-1-willy@infradead.org
> 
> If the driver is not in the tree, people don't care.
> 
> You really should try upstreaming that driver.
> 
> 
> So this patch here adds complexity (which I don't like) in order to keep an
> OOT driver working -- possibly for a short time. I'm tempted to say "please
> fix your driver to not use huge faults in that scenario, it is no longer
> supported".
> 
> But I'm just about to vanish for 1.5 week into vacation :)
> 
> @Willy, what are your thoughts?

Fundamentally there was a bad assumption with the original patch --
it assumed that the only reason to support ->huge_fault was for DAX,
and that's not true.  It's just that the only drivers in-tree which
support ->huge_fault do so in order to support DAX.

Keeping a driver out of tree is always a risky and costly proposition.
It will continue to be broken by core kernel changes, particularly
if/when it does unusual things.

I think the complexity is entirely on us.  I think there's a simpler way
to handle the problem, but I'd start by turning all of this "admin and
app get to control when THP are used" nonsense into no-ops.