linux-ext4 - Re: [PATCH 07/12] mm: Remove redundant pXd

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Message-ID: <799691d6-d9dd-43fd-805e-7dce6efff3e6@redhat.com>
Date: Mon, 2 Jun 2025 14:20:52 +0200
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org
Cc: gerald.schaefer@...ux.ibm.com, dan.j.williams@...el.com, jgg@...pe.ca,
 willy@...radead.org, linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
 linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
 linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
 zhang.lyra@...il.com, debug@...osinc.com, bjorn@...nel.org,
 balbirs@...dia.com, lorenzo.stoakes@...cle.com,
 linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-cxl@...r.kernel.org, dri-devel@...ts.freedesktop.org, John@...ves.net
Subject: Re: [PATCH 07/12] mm: Remove redundant pXd_devmap calls

On 02.06.25 11:33, David Hildenbrand wrote:
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1398,10 +1398,7 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>>    	}
>>    
>>    	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
>> -	if (pfn_t_devmap(pfn))
>> -		entry = pmd_mkdevmap(entry);
>> -	else
>> -		entry = pmd_mkspecial(entry);
>> +	entry = pmd_mkspecial(entry);
>>    	if (write) {
> 
> 
> I just stumbled over this, and I think there is something off here in
> the PMD/PUD case.
> 
> vmf_insert_folio_pmd() does a folio_get() + folio_add_file_rmap_pmd().
> 
> But then, we go ahead and turn this into a special mapping by setting it
> pmd_mkdevmap()/pmd_mkspecial().
> 
> Consequently, vm_normal_page_pmd() would ignore them, not following the
> rules documented for vm_normal_page() and behaving differently than
> vmf_insert_page_mkwrite()->insert_page().
> 
> 
> folio_add_file_rmap_pmd() should never set these things special/devmap
> in the first place :/
> 
> What am I missing?
> 
> Note that fs/dax.c calls vmf_insert_folio_pmd() for PMDs and
> vmf_insert_page_mkwrite() for PTEs.
> 
> Consequently, PTEs will never be marked special (corner case, shared
> zeropage), but PMDs would always.
> 
> Hm?

What an ugly piece of crap this pmd handling code is.

Just noting because I stumbled over that myself:

pmd_mkdevmap() does *not* imply pmd_special().

... in contrast to pte_mkdevmap(), which will imply pte_special().


-- 
Cheers,

David / dhildenb