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

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Message-ID: <88060373-d0b8-404a-b53b-9b5a54f5ba6f@linux.alibaba.com>
Date: Wed, 7 May 2025 20:36:54 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, david@...hat.com, 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 2025/5/7 20:10, 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?

I did mm selftests, tmpfs/xfs PMD-sized mmap() tests.

> 
>> -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'.

Please look at the following code in do_set_pmd(), and the 'page' will 
be initialized before using.

         if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags))
                 return ret;

         if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
                 return ret;

         if (folio_order(folio) != HPAGE_PMD_ORDER)
                 return ret;
         page = &folio->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

No, I have no this assumption. But do_set_pmd() is used to establish PMD 
mappings for the PMD-sized folios, and we already have PMD-sized checks 
to validate the folio size:

         if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
                 return ret;

         if (folio_order(folio) != HPAGE_PMD_ORDER)
                 return ret;

> 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

IMO, for PMD mapping of a PMD-sized folio, we do not need to know 
_which_ page in the folio we want to map, because we'll always map the 
entire PMD-sized folio.

> the vmf, NACK this patch.