From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx168.postini.com [74.125.245.168]) by kanga.kvack.org (Postfix) with SMTP id A17706B004D for ; Mon, 30 Jan 2012 01:23:47 -0500 (EST) Received: from m4.gw.fujitsu.co.jp (unknown [10.0.50.74]) by fgwmail6.fujitsu.co.jp (Postfix) with ESMTP id 8C67B3EE0B6 for ; Mon, 30 Jan 2012 15:23:45 +0900 (JST) Received: from smail (m4 [127.0.0.1]) by outgoing.m4.gw.fujitsu.co.jp (Postfix) with ESMTP id 6E77645DE50 for ; Mon, 30 Jan 2012 15:23:45 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (s4.gw.fujitsu.co.jp [10.0.50.94]) by m4.gw.fujitsu.co.jp (Postfix) with ESMTP id 4CDB945DE4D for ; Mon, 30 Jan 2012 15:23:45 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id 1BBEE1DB803B for ; Mon, 30 Jan 2012 15:23:45 +0900 (JST) Received: from m107.s.css.fujitsu.com (m107.s.css.fujitsu.com [10.240.81.147]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id BB7391DB802F for ; Mon, 30 Jan 2012 15:23:44 +0900 (JST) Date: Mon, 30 Jan 2012 15:22:12 +0900 From: KAMEZAWA Hiroyuki Subject: Re: [PATCH 2/6] thp: optimize away unnecessary page table locking Message-Id: <20120130152212.3a6a2039.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <1327705373-29395-3-git-send-email-n-horiguchi@ah.jp.nec.com> References: <1327705373-29395-1-git-send-email-n-horiguchi@ah.jp.nec.com> <1327705373-29395-3-git-send-email-n-horiguchi@ah.jp.nec.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Naoya Horiguchi Cc: linux-mm@kvack.org, Andrew Morton , David Rientjes , Andi Kleen , Wu Fengguang , Andrea Arcangeli , KOSAKI Motohiro , linux-kernel@vger.kernel.org On Fri, 27 Jan 2012 18:02:49 -0500 Naoya Horiguchi wrote: > Currently when we check if we can handle thp as it is or we need to > split it into regular sized pages, we hold page table lock prior to > check whether a given pmd is mapping thp or not. Because of this, > when it's not "huge pmd" we suffer from unnecessary lock/unlock overhead. > To remove it, this patch introduces a optimized check function and > replace several similar logics with it. > > Signed-off-by: Naoya Horiguchi > Cc: David Rientjes > > Changes since v3: > - Fix likely/unlikely pattern in pmd_trans_huge_stable() > - Change suffix from _stable to _lock > - Introduce __pmd_trans_huge_lock() to avoid micro-regression > - Return 1 when wait_split_huge_page path is taken > > Changes since v2: > - Fix missing "return 0" in "thp under splitting" path > - Remove unneeded comment > - Change the name of check function to describe what it does > - Add VM_BUG_ON(mmap_sem) > +/* > + * Returns 1 if a given pmd maps a stable (not under splitting) thp, > + * -1 if the pmd maps thp under splitting, 0 if the pmd does not map thp. > + * > + * Note that if it returns 1, this routine returns without unlocking page > + * table locks. So callers must unlock them. > + */ Seems nice clean up but... why you need to return (-1, 0, 1) ? It seems the caller can't see the difference between -1 and 0. Why not just return 0 (not locked) or 1 (thp found and locked) ? Thanks, -Kame > +int __pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma) > +{ > + spin_lock(&vma->vm_mm->page_table_lock); > if (likely(pmd_trans_huge(*pmd))) { > if (unlikely(pmd_trans_splitting(*pmd))) { > - spin_unlock(&mm->page_table_lock); > + spin_unlock(&vma->vm_mm->page_table_lock); > wait_split_huge_page(vma->anon_vma, pmd); > + return -1; > } else { > - pmd_t entry; > - > - entry = pmdp_get_and_clear(mm, addr, pmd); > - entry = pmd_modify(entry, newprot); > - set_pmd_at(mm, addr, pmd, entry); > - spin_unlock(&vma->vm_mm->page_table_lock); > - ret = 1; > + /* Thp mapped by 'pmd' is stable, so we can > + * handle it as it is. */ > + return 1; > } > - } else > - spin_unlock(&vma->vm_mm->page_table_lock); > - > - return ret; > + } > + spin_unlock(&vma->vm_mm->page_table_lock); > + return 0; > } > > pmd_t *page_check_address_pmd(struct page *page, > -- > 1.7.7.6 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ > Don't email: email@kvack.org > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org