linux-kernel - Re: [PATCHv3 03/24] mm: avoid PG

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Date:	Thu, 12 Feb 2015 14:55:58 -0500
From:	Rik van Riel <riel@...hat.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
CC:	Dave Hansen <dave.hansen@...el.com>, Mel Gorman <mgorman@...e.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 03/24] mm: avoid PG_locked on tail pages

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On 02/12/2015 11:18 AM, Kirill A. Shutemov wrote:
> With new refcounting pte entries can point to tail pages. It's
> doesn't make much sense to mark tail page locked -- we need to
> protect whole compound page.
> 
> This patch adjust helpers related to PG_locked to operate on head
> page.
> 
> Signed-off-by: Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> --- include/linux/page-flags.h |
> 3 ++- include/linux/pagemap.h    | 5 +++++ mm/filemap.c
> | 1 + mm/slub.c                  | 2 ++ 4 files changed, 10
> insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/page-flags.h
> b/include/linux/page-flags.h index 5ed7bdaf22d5..d471370f27e8
> 100644 --- a/include/linux/page-flags.h +++
> b/include/linux/page-flags.h @@ -207,7 +207,8 @@ static inline int
> __TestClearPage##uname(struct page *page) { return 0; }
> 
> struct page;	/* forward declaration */
> 
> -TESTPAGEFLAG(Locked, locked) +#define PageLocked(page)
> test_bit(PG_locked, &compound_head(page)->flags) + PAGEFLAG(Error,
> error) TESTCLEARFLAG(Error, error) PAGEFLAG(Referenced, referenced)
> TESTCLEARFLAG(Referenced, referenced) __SETPAGEFLAG(Referenced,
> referenced) diff --git a/include/linux/pagemap.h
> b/include/linux/pagemap.h index 4b3736f7065c..ad6da4e49555 100644 
> --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@
> -428,16 +428,19 @@ extern void unlock_page(struct page *page);
> 
> static inline void __set_page_locked(struct page *page) { +
> VM_BUG_ON_PAGE(PageTail(page), page); __set_bit(PG_locked,
> &page->flags); }
> 
> static inline void __clear_page_locked(struct page *page) { +
> VM_BUG_ON_PAGE(PageTail(page), page); __clear_bit(PG_locked,
> &page->flags); }
> 
> static inline int trylock_page(struct page *page) { +	page =
> compound_head(page); return
> (likely(!test_and_set_bit_lock(PG_locked, &page->flags))); }
> 
> @@ -490,6 +493,7 @@ extern int
> wait_on_page_bit_killable_timeout(struct page *page,
> 
> static inline int wait_on_page_locked_killable(struct page *page) 
> { +	page = compound_head(page); if (PageLocked(page)) return
> wait_on_page_bit_killable(page, PG_locked); return 0; @@ -510,6
> +514,7 @@ static inline void wake_up_page(struct page *page, int
> bit) */ static inline void wait_on_page_locked(struct page *page) 
> { +	page = compound_head(page); if (PageLocked(page)) 
> wait_on_page_bit(page, PG_locked); }

These are all atomic operations.

This may be a stupid question with the answer lurking somewhere
in the other patches, but how do you ensure you operate on the
right page lock during a THP collapse or split?



- -- 
All rights reversed
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