linux-kernel - Re: [PATCH] rmap: add exclusively owned pages to the newest anon

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Date:	Thu, 15 Apr 2010 13:01:11 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
cc:	Borislav Petkov <bp@...en8.de>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com
Subject: Re: [PATCH] rmap: add exclusively owned pages to the newest
 anon_vma



On Wed, 14 Apr 2010, Rik van Riel wrote:
> -	/*
> -	 * We must use the _oldest_ possible anon_vma for the page mapping!
> -	 *
> -	 * So take the last AVC chain entry in the vma, which is the deepest
> -	 * ancestor, and use the anon_vma from that.
> -	 */
> -	avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
> -	anon_vma = avc->anon_vma;
> +	if (exclusive)
> +		anon_vma = vma->anon_vma;
> +	else {
> +		/*
> +		 * The page may be shared between multiple processes.
> +		 * We must use the _oldest_ possible anon_vma for the
> +		 * page mapping!  That anon_vma is guaranteed to be
> +		 * present in all processes that could share this page.
> +		 *
> +		 * So take the last AVC chain entry in the vma, which is the
> +		 * deepest ancestor, and use the anon_vma from that.
> +		 */
> +		avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
> +		anon_vma = avc->anon_vma;
> +	}

I really dislike your coding style.

If we do this conditionally, we're _much_ better off declaring the 
variables we only use inside that conditional block inside the block 
itself. And since we access "vma->anon_vma" in either case, just move that 
case outside the conditional statement, and avoid a pointless 
if/then/else.

IOW, something like this. Totally untested.

		Linus

---
 mm/rmap.c |   26 +++++++++++++++-----------
 1 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 4bad326..78d4730 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -732,21 +732,25 @@ void page_move_anon_rmap(struct page *page,
  * @address:	the user virtual address mapped
  */
 static void __page_set_anon_rmap(struct page *page,
-	struct vm_area_struct *vma, unsigned long address)
+	struct vm_area_struct *vma, unsigned long address, int exclusive)
 {
-	struct anon_vma_chain *avc;
-	struct anon_vma *anon_vma;
+	struct anon_vma *anon_vma = vma->anon_vma;
 
-	BUG_ON(!vma->anon_vma);
+	BUG_ON(!anon_vma);
 
 	/*
-	 * We must use the _oldest_ possible anon_vma for the page mapping!
+	 * If the page isn't exclusively mapped into this vma,
+	 * we must use the _oldest_ possible anon_vma for the
+	 * page mapping!
 	 *
-	 * So take the last AVC chain entry in the vma, which is the deepest
-	 * ancestor, and use the anon_vma from that.
+	 * So take the last AVC chain entry in the vma, which is
+	 * the deepest ancestor, and use the anon_vma from that.
 	 */
-	avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
-	anon_vma = avc->anon_vma;
+	if (!exclusive) {
+		struct anon_vma_chain *avc;
+		avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
+		anon_vma = avc->anon_vma;
+	}
 
 	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
 	page->mapping = (struct address_space *) anon_vma;
@@ -802,7 +806,7 @@ void page_add_anon_rmap(struct page *page,
 	VM_BUG_ON(!PageLocked(page));
 	VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
 	if (first)
-		__page_set_anon_rmap(page, vma, address);
+		__page_set_anon_rmap(page, vma, address, 0);
 	else
 		__page_check_anon_rmap(page, vma, address);
 }
@@ -824,7 +828,7 @@ void page_add_new_anon_rmap(struct page *page,
 	SetPageSwapBacked(page);
 	atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */
 	__inc_zone_page_state(page, NR_ANON_PAGES);
-	__page_set_anon_rmap(page, vma, address);
+	__page_set_anon_rmap(page, vma, address, 1);
 	if (page_evictable(page, vma))
 		lru_cache_add_lru(page, LRU_ACTIVE_ANON);
 	else
--
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