linux-kernel - Re: [PATCH 2/2] mm: convert do_set

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Message-ID: <d2348cc3-c9c6-4df0-82b6-1105edd44a75@redhat.com>
Date: Wed, 7 May 2025 23:24:01 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, ziy@...dia.com, vbabka@...e.cz,
 rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: convert do_set_pmd() to take a folio

On 07.05.25 14:10, Matthew Wilcox wrote:
> On Wed, May 07, 2025 at 05:26:13PM +0800, Baolin Wang wrote:
>> In do_set_pmd(), we always use the folio->page to build PMD mappings for
>> the entire folio. Since all callers of do_set_pmd() already hold a stable
>> folio, converting do_set_pmd() to take a folio is safe and more straightforward.
> 
> What testing did you do of this?
> 
>> -vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
>> +vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio)
>>   {
>> -	struct folio *folio = page_folio(page);
>>   	struct vm_area_struct *vma = vmf->vma;
>>   	bool write = vmf->flags & FAULT_FLAG_WRITE;
>>   	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>>   	pmd_t entry;
>>   	vm_fault_t ret = VM_FAULT_FALLBACK;
>> +	struct page *page;
> 
> Because I see nowhere in this patch that you initialise 'page'.
> 
> And that's really the important part.  You seem to be assuming that a
> folio will never be larger than PMD size, and I'm not comfortable with
> that assumption.  It's a limitation I put in place a few years ago so we
> didn't have to find and fix all those assumptions immediately, but I
> imagine that some day we'll want to have larger folios.
> 
> So unless you can derive _which_ page in the folio we want to map from
> the vmf, NACK this patch.

Agreed. Probably folio + idx is our best bet.

Which raises an interesting question: I assume in the future, when we 
have a 4 MiB folio on x86-64 that is *misaligned* in VA space regarding 
PMDs (e.g., aligned to 1 MiB but not 2 MiB), we could still allow to use 
a PMD for the middle part.

So idx must not necessarily be aligned to PMDs in the future.

For now, we could sanity-check that idx is always 0.

But the rmap sanity checks in folio_add_file_rmap_pmd() will already 
catch that for us.

-- 
Cheers,

David / dhildenb