linux-kernel - Re: [PATCH 22/23] thp, mm: split huge page on mmap file page

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Date:	Thu, 08 Aug 2013 14:49:27 -0600
From:	Khalid Aziz <khalid.aziz@...cle.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, Dave Hansen <dave@...1.net>,
	Ning Qu <quning@...gle.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 22/23] thp, mm: split huge page on mmap file page

On Sun, 2013-08-04 at 05:17 +0300, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> 
> We are not ready to mmap file-backed tranparent huge pages. Let's split
> them on fault attempt.
> 
> Later we'll implement mmap() properly and this code path be used for
> fallback cases.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  mm/filemap.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index ed65af5..f7857ef 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1743,6 +1743,8 @@ retry_find:
>  			goto no_cached_page;
>  	}
>  
> +	if (PageTransCompound(page))
> +		split_huge_page(compound_trans_head(page));

Since PageTransCompound(page) returns true for transparent huge pages as
well as hugetlbfs pages, could this code split hugetlbfs pages on an
mmap() on to hugetlbfs pages? hugetlbfs pages are not supposed to be
split, right?

>  	if (!lock_page_or_retry(page, vma->vm_mm, vmf->flags)) {
>  		page_cache_release(page);
>  		return ret | VM_FAULT_RETRY;

--
Khalid

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