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

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Message-ID: <0fd1bf14-a664-49c0-a239-fdc45b8e3a39@redhat.com>
Date: Thu, 8 May 2025 09:37:08 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Matthew Wilcox <willy@...radead.org>
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 08.05.25 04:23, Baolin Wang wrote:
> 
> 
> On 2025/5/8 00:47, Matthew Wilcox wrote:
>> On Wed, May 07, 2025 at 08:36:54PM +0800, Baolin Wang wrote:
>>> On 2025/5/7 20:10, Matthew Wilcox wrote:
>>>> 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;
>>
>> Ah, fair, I missed that.
>>
>>>> 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.
>>
>> There's a difference between "Assert that the folio is PMD sized" inside
>> the function because we know there are still problems, and "Change the
>> interface so we can't specify which page inside the folio is the one
>> we're actually interested in".
> 
> Fair enough. So how about adding a new 'folio' parameter to
> do_set_pmd(), similar to the set_pte_range() function prototype?
> 
> vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct
> page *page)

That's what I used for rmap functions. *Maybe* folio+idx is better: 
might avoid having to lookup the page in some cases (probably in the 
future).

-- 
Cheers,

David / dhildenb