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Date: Mon, 9 Sep 2024 16:15:39 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Peter Xu <peterx@redhat.com>
Cc: Yan Zhao <yan.y.zhao@intel.com>, linux-kernel@vger.kernel.org,
 linux-mm@kvack.org, Gavin Shan <gshan@redhat.com>, Catalin Marinas
 <catalin.marinas@arm.com>, x86@kernel.org, Ingo Molnar <mingo@redhat.com>,
 Paolo Bonzini <pbonzini@redhat.com>, Dave Hansen
 <dave.hansen@linux.intel.com>, Thomas Gleixner <tglx@linutronix.de>,
 Alistair Popple <apopple@nvidia.com>, kvm@vger.kernel.org,
 linux-arm-kernel@lists.infradead.org, Sean Christopherson
 <seanjc@google.com>, Oscar Salvador <osalvador@suse.de>, Jason Gunthorpe
 <jgg@nvidia.com>, Borislav Petkov <bp@alien8.de>, Zi Yan <ziy@nvidia.com>,
 Axel Rasmussen <axelrasmussen@google.com>, David Hildenbrand
 <david@redhat.com>, Will Deacon <will@kernel.org>, Kefeng Wang
 <wangkefeng.wang@huawei.com>, Alex Williamson <alex.williamson@redhat.com>
Subject: Re: [PATCH v2 07/19] mm/fork: Accept huge pfnmap entries
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On Mon, 9 Sep 2024 18:43:22 -0400 Peter Xu <peterx@redhat.com> wrote:

> > > > Do we need the logic to clear dirty bit in the child as that in
> > > > __copy_present_ptes()?  (and also for the pmd's case).
> > > > 
> > > > e.g.
> > > > if (vma->vm_flags & VM_SHARED)
> > > > 	pud = pud_mkclean(pud);
> > > 
> > > Yeah, good question.  I remember I thought about that when initially
> > > working on these lines, but I forgot the details, or maybe I simply tried
> > > to stick with the current code base, as the dirty bit used to be kept even
> > > in the child here.
> > > 
> > > I'd expect there's only performance differences, but still sounds like I'd
> > > better leave that to whoever knows the best on the implications, then draft
> > > it as a separate patch but only when needed.
> > 
> > Sorry, but this vaguensss simply leaves me with nowhere to go.
> > 
> > I'll drop the series - let's revisit after -rc1 please.
> 
> Andrew, would you please explain why it needs to be dropped?
> 
> I meant in the reply that I think we should leave that as is, and I think
> so far nobody in real life should care much on this bit, so I think it's
> fine to leave the dirty bit as-is.
> 
> I still think whoever has a better use of the dirty bit and would like to
> change the behavior should find the use case and work on top, but only if
> necessary.

Well.  "I'd expect there's only performance differences" means to me
"there might be correctness issues, I don't know".  Is it or is it not
merely a performance thing?