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

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Message-ID: <ZsL_XvRL-OgEqmvv@google.com>
Date: Mon, 19 Aug 2024 09:16:30 +0100
From: Vincent Donnefort <vdonnefort@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: 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 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:
> > On Tue, 11 Jun 2024 17:33:17 +0200 David Hildenbrand <david@...hat.com> wrote:
> > 
> > > On 11.06.24 17:32, Andrew Bresticker wrote:
> > > > The requirement that the head page be passed to do_set_pmd() was added
> > > > in commit ef37b2ea08ac ("mm/memory: page_add_file_rmap() ->
> > > > folio_add_file_rmap_[pte|pmd]()") and prevents pmd-mapping in the
> > > > finish_fault() and filemap_map_pages() paths if the page to be inserted
> > > > is anything but the head page for an otherwise suitable vma and pmd-sized
> > > > page.
> > > > 
> > > > Fixes: ef37b2ea08ac ("mm/memory: page_add_file_rmap() -> folio_add_file_rmap_[pte|pmd]()")
> > > > Signed-off-by: Andrew Bresticker <abrestic@...osinc.com>
> > > > ---
> > > >   mm/memory.c | 3 ++-
> > > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index 0f47a533014e..a1fce5ddacb3 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -4614,8 +4614,9 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
> > > >   	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
> > > >   		return ret;
> > > >   
> > > > -	if (page != &folio->page || folio_order(folio) != HPAGE_PMD_ORDER)
> > > > +	if (folio_order(folio) != HPAGE_PMD_ORDER)
> > > >   		return ret;
> > > > +	page = &folio->page;
> > > >   
> > > >   	/*
> > > >   	 * Just backoff if any subpage of a THP is corrupted otherwise
> > > 
> > > Acked-by: David Hildenbrand <david@...hat.com>
> > 
> > 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.