From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753870Ab2A3GXs (ORCPT ); Mon, 30 Jan 2012 01:23:48 -0500 Received: from fgwmail5.fujitsu.co.jp ([192.51.44.35]:35252 "EHLO fgwmail5.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752642Ab2A3GXr (ORCPT ); Mon, 30 Jan 2012 01:23:47 -0500 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.3.1 Date: Mon, 30 Jan 2012 15:22:12 +0900 From: KAMEZAWA Hiroyuki 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 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> Organization: FUJITSU Co. LTD. X-Mailer: Sylpheed 3.1.1 (GTK+ 2.10.14; i686-pc-mingw32) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: 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 >