Re: [PATCH 2/2] mm: convert do_set_pmd() to take a folio

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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)

> I reiterate the NACK to this patch.