linux-kernel - Re: [PATCH v2 07/19] mm/fork: Accept huge pfnmap entries

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Message-ID: <ZtC9ThIs7aSK7gdK@x1n>
Date: Thu, 29 Aug 2024 14:26:22 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Gavin Shan <gshan@...hat.com>,
	Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alistair Popple <apopple@...dia.com>, kvm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Sean Christopherson <seanjc@...gle.com>,
	Oscar Salvador <osalvador@...e.de>,
	Jason Gunthorpe <jgg@...dia.com>, Borislav Petkov <bp@...en8.de>,
	Zi Yan <ziy@...dia.com>, Axel Rasmussen <axelrasmussen@...gle.com>,
	Yan Zhao <yan.y.zhao@...el.com>, Will Deacon <will@...nel.org>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v2 07/19] mm/fork: Accept huge pfnmap entries

On Thu, Aug 29, 2024 at 05:10:42PM +0200, David Hildenbrand wrote:
> On 26.08.24 22:43, Peter Xu wrote:
> > Teach the fork code to properly copy pfnmaps for pmd/pud levels.  Pud is
> > much easier, the write bit needs to be persisted though for writable and
> > shared pud mappings like PFNMAP ones, otherwise a follow up write in either
> > parent or child process will trigger a write fault.
> > 
> > Do the same for pmd level.
> > 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >   mm/huge_memory.c | 29 ++++++++++++++++++++++++++---
> >   1 file changed, 26 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index e2c314f631f3..15418ffdd377 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1559,6 +1559,24 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> >   	pgtable_t pgtable = NULL;
> >   	int ret = -ENOMEM;
> > +	pmd = pmdp_get_lockless(src_pmd);
> > +	if (unlikely(pmd_special(pmd))) {
> 
> I assume I have to clean up your mess here as well?

Can you leave meaningful and explicit comment?  I'll try to address.

-- 
Peter Xu