linux-kernel - Re: [PATCH v2] mm/memory: Don't require head page for do_set

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Message-ID: <0d4f99bd-609a-8c76-b665-adcf942e6a74@google.com>
Date: Tue, 20 Aug 2024 13:33:59 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Vincent Donnefort <vdonnefort@...gle.com>
cc: Matthew Wilcox <willy@...radead.org>, 
    Andrew Morton <akpm@...ux-foundation.org>, 
    David Hildenbrand <david@...hat.com>, 
    Andrew Bresticker <abrestic@...osinc.com>, linux-mm@...ck.org, 
    linux-kernel@...r.kernel.org, maz@...nel.org, seanjc@...gle.com
Subject: Re: [PATCH v2] mm/memory: Don't require head page for do_set_pmd()

On Mon, 19 Aug 2024, Vincent Donnefort wrote:
> On Tue, Jun 11, 2024 at 07:22:03PM +0100, Matthew Wilcox wrote:
> > On Tue, Jun 11, 2024 at 11:06:22AM -0700, Andrew Morton wrote:
...
> > > You know what I'm going to ask ;) I'm assuming that the runtime effects
> > > are "small performance optimization" and that "should we backport the
> > > fix" is "no".
> > 
> > We're going to stop using PMDs to map large folios unless the fault is
> > within the first 4KiB of the PMD.  No idea how many workloads that
> > affects, but it only needs to be backported as far as v6.8, so we
> > may as well backport it.
> 
> Hi, I am reviving this thread after noticing this comment attached
> to the fix.
> 
> If you intend to install PTE level mappings for faults that happen outside of
> the first 4KiB, I believe this will make THP support for KVM ineffective.

You can relax, it's okay: where Matthew wrote "We're going to stop...",
he was describing the runtime effects of the bug (now fixed) to Andrew,
not proposing to make a change to mess up THP support.

The fix was backported to v6.9.N, but was too late for v6.8.N EOL.

Hugh