linux-kernel - Re: [PATCH 1/2] mm/page_vma_mapped: use PMD

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Date:   Thu, 28 Nov 2019 21:22:26 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Wei Yang <richardw.yang@...ux.intel.com>,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/page_vma_mapped: use PMD_SIZE instead of
 calculating it

On Thu, Nov 28, 2019 at 11:32:55AM +0300, Kirill A. Shutemov wrote:
>On Thu, Nov 28, 2019 at 09:03:20AM +0800, Wei Yang wrote:
>> At this point, we are sure page is PageTransHuge, which means
>> hpage_nr_pages is HPAGE_PMD_NR.
>> 
>> This is safe to use PMD_SIZE instead of calculating it.
>> 
>> Signed-off-by: Wei Yang <richardw.yang@...ux.intel.com>
>> ---
>>  mm/page_vma_mapped.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>> index eff4b4520c8d..76e03650a3ab 100644
>> --- a/mm/page_vma_mapped.c
>> +++ b/mm/page_vma_mapped.c
>> @@ -223,7 +223,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>  			if (pvmw->address >= pvmw->vma->vm_end ||
>>  			    pvmw->address >=
>>  					__vma_address(pvmw->page, pvmw->vma) +
>> -					hpage_nr_pages(pvmw->page) * PAGE_SIZE)
>> +					PMD_SIZE)
>>  				return not_found(pvmw);
>>  			/* Did we cross page table boundary? */
>>  			if (pvmw->address % PMD_SIZE == 0) {
>
>It is dubious cleanup. Maybe page_size(pvmw->page) instead? It will not
>break if we ever get PUD THP pages.
>

Thanks for your comment.

I took a look into the code again and found I may miss something.

I found we support PUD THP pages, whilc hpage_nr_pages() just return
HPAGE_PMD_NR on PageTransHuge. Why this is not possible to return PUD number?

Search in the kernel tree, one implementation of PUD_SIZE fault is
dev_dax_huge_fault. If page fault happens here, would this if break the loop
too early?

>-- 
> Kirill A. Shutemov

-- 
Wei Yang
Help you, Help me