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From: Matthew Wilcox <willy@infradead.org>
To: Yang Shi <shy828301@gmail.com>
Cc: songmuchun@bytedance.com, akpm@linux-foundation.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] mm: pvmw: check possible huge PMD map by transhuge_vma_suitable()
Date: Fri, 13 May 2022 04:30:52 +0100	[thread overview]
Message-ID: <Yn3Q7KIb3FcAPW9j@casper.infradead.org> (raw)
In-Reply-To: <20220512174551.81279-2-shy828301@gmail.com>

On Thu, May 12, 2022 at 10:45:51AM -0700, Yang Shi wrote:
> IIUC PVMW checks if the vma is possibly huge PMD mapped by
> transparent_hugepage_active() and "pvmw->nr_pages >= HPAGE_PMD_NR".
> 
> Actually pvmw->nr_pages is returned by compound_nr() or
> folio_nr_pages(), so the page should be THP as long as "pvmw->nr_pages
> >= HPAGE_PMD_NR".  And it is guaranteed THP is allocated for valid VMA
> in the first place.  But it may be not PMD mapped if the VMA is file
> VMA and it is not properly aligned.  The transhuge_vma_suitable()
> is used to do such check, so replace transparent_hugepage_active() to
> it, which is too heavy and overkilling.
> 
> Fixes: 2aff7a4755be ("mm: Convert page_vma_mapped_walk to work on PFNs")

I think Fixes is a bit much.  There's no bug being fixed here.  This is
just an optimisation.  Is it an important optimisation?  We could put a
bool into page_vma_mapped_walk() so we only have to ask the page whether
it's PMD-mappable once per walk rather than for each VMA.


  parent reply	other threads:[~2022-05-13  3:31 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-12 17:45 [PATCH 1/2] mm: rmap: use the correct parameter name for DEFINE_PAGE_VMA_WALK Yang Shi
2022-05-12 17:45 ` [PATCH 2/2] mm: pvmw: check possible huge PMD map by transhuge_vma_suitable() Yang Shi
2022-05-12 23:39   ` Andrew Morton
2022-05-13  0:39     ` Yang Shi
2022-05-13  1:41   ` kernel test robot
2022-05-13  3:30   ` Matthew Wilcox [this message]
2022-05-13  3:51     ` Yang Shi
2022-05-13  2:16 ` [PATCH 1/2] mm: rmap: use the correct parameter name for DEFINE_PAGE_VMA_WALK Matthew Wilcox
2022-05-13  2:42 ` Muchun Song

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