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

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Message-ID: <32a451ee-6836-4d4d-814c-752c15415aae@redhat.com>
Date: Thu, 29 Aug 2024 21:44:01 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...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 29.08.24 20:26, Peter Xu wrote:
> 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.

Sorry Peter, but I raised all that as reply to v1. For example, I 
stated that vm_normal_page_pmd() already *exist* and why these 
pmd_special() checks should be kept there.

I hear you, you're not interested in cleaning that up. So at this point 
it's easier for me to clean it up myself.

-- 
Cheers,

David / dhildenb