linux-kernel - Re: [PATCH 05/19] mm/gup: Detect huge pfnmap entries in gup-fast

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Message-ID: <ZrZK_Pk1fMGUCLUN@x1n>
Date: Fri, 9 Aug 2024 12:59:40 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Sean Christopherson <seanjc@...gle.com>,
	Oscar Salvador <osalvador@...e.de>,
	Jason Gunthorpe <jgg@...dia.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
	Will Deacon <will@...nel.org>, Gavin Shan <gshan@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Zi Yan <ziy@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Alistair Popple <apopple@...dia.com>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Yan Zhao <yan.y.zhao@...el.com>
Subject: Re: [PATCH 05/19] mm/gup: Detect huge pfnmap entries in gup-fast

On Fri, Aug 09, 2024 at 06:23:53PM +0200, David Hildenbrand wrote:
> On 09.08.24 18:08, Peter Xu wrote:
> > Since gup-fast doesn't have the vma reference, teach it to detect such huge
> > pfnmaps by checking the special bit for pmd/pud too, just like ptes.
> > 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >   mm/gup.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index d19884e097fd..a49f67a512ee 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -3038,6 +3038,9 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> >   	if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
> >   		return 0;
> > +	if (pmd_special(orig))
> > +		return 0;
> > +
> >   	if (pmd_devmap(orig)) {
> >   		if (unlikely(flags & FOLL_LONGTERM))
> >   			return 0;
> > @@ -3082,6 +3085,9 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
> >   	if (!pud_access_permitted(orig, flags & FOLL_WRITE))
> >   		return 0;
> > +	if (pud_special(orig))
> > +		return 0;
> > +
> >   	if (pud_devmap(orig)) {
> >   		if (unlikely(flags & FOLL_LONGTERM))
> >   			return 0;
> 
> In gup_fast_pte_range() we check after checking pte_devmap(). Do we want to
> do it in a similar fashion here, or is there a reason to do it differently?

IIUC they should behave the same, as the two should be mutual exclusive so
far.  E.g. see insert_pfn():

	if (pfn_t_devmap(pfn))
		entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
	else
		entry = pte_mkspecial(pfn_t_pte(pfn, prot));

It might change for sure if Alistair move on with the devmap work, though..
these two always are processed together now, so I hope that won't add much
burden which series will land first, then we may need some care on merging
them.  I don't expect anything too tricky in merge if that was about
removal of the devmap bits.

> 
> Acked-by: David Hildenbrand <david@...hat.com>

Thanks, I'll take this one first.

-- 
Peter Xu