From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5224CC3DA7A for ; Mon, 2 Jan 2023 12:04:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232490AbjABMEZ (ORCPT ); Mon, 2 Jan 2023 07:04:25 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38756 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230196AbjABMEW (ORCPT ); Mon, 2 Jan 2023 07:04:22 -0500 Received: from mail-pl1-x633.google.com (mail-pl1-x633.google.com [IPv6:2607:f8b0:4864:20::633]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6B7AE9F for ; Mon, 2 Jan 2023 04:04:19 -0800 (PST) Received: by mail-pl1-x633.google.com with SMTP id y19so10352781plb.2 for ; Mon, 02 Jan 2023 04:04:19 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=SWkL+Ay7xGJduzHxgREoGWnEpK2FebgVdJq9e4wRN74=; b=IB119u7Sbb6z/UYOkoKjjjy1GSSLRU6TTtuM8wlBc1pyTrPjYczTgibOYnNMUCH/tO 1pmS862dpdi6fTEQ14TAz5hwKSjXCotOrLE1Nc6RuILhfzEgp33izj0hbu0tq5+gMDVS O9UizHTR1Pwa9ngdLj1WLxyk/VmTdphDL4YuTxISO8MbXy5OnSoBphVgv2rsTyOygInd u58lgi20ws/UweMAU4HzBlvMvDwPTjzfeyMS8o61dO+cpGwxKFLtjqkmIMjc/oxX71yo N0dLWB312Nw0kTAijIWE87GDVT1rHdji37xM8Z4w4/N6jtNQbwrmKUILgZH7LSUoEcXr P31g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=SWkL+Ay7xGJduzHxgREoGWnEpK2FebgVdJq9e4wRN74=; b=m8bwQAt22Pi8bBzMPbVZiNOfbjEo/EA3xRV/yQcRz9cr+TrP+o9ZEmpj9de/Wst0pl FTWHB5MYiyC90onqy0p1aAZ2aBqmzDuuaz2yL2MKnBgZ7n++5xWNPgo9r3pj6bcWYpOX 8cROeOq1nA1KwaYRuDZIXZV353pFmu1h3wh+MegPH++NnF332ajCHtkFTnVLkO5McaM6 isVtSirBnHT99hJOAK5Xu6DjP8Bbgx2YM8fhA0ofUe5lXbEichIS+XPt9vA3cdPXX0Vw vp53VTCuckYCRt5wf23JT6URhbl+PMBhBPej4mmVDyt0oSWmkW5V/9DsW3xiUHRddaU0 0KfQ== X-Gm-Message-State: AFqh2kp+QH3AG5BmEe2uKuCfTB8Z1Ymd0uRxCks5z7AosiMBXmLF+wus 9PW2awpd9apWCHvtRv1l9vo= X-Google-Smtp-Source: AMrXdXsdAtxobuHHkRUmvju928j1cQhZ7HR9IzeQhw6EW7+Rqjzk/ZZpSduHYIKwxIlyVS7+qjkPsg== X-Received: by 2002:a17:903:2792:b0:192:8c1a:1bea with SMTP id jw18-20020a170903279200b001928c1a1beamr24377662plb.43.1672661058789; Mon, 02 Jan 2023 04:04:18 -0800 (PST) Received: from hyeyoo ([114.29.91.56]) by smtp.gmail.com with ESMTPSA id h6-20020a63c006000000b00478b930f970sm16671838pgg.66.2023.01.02.04.04.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Jan 2023 04:04:17 -0800 (PST) Date: Mon, 2 Jan 2023 21:04:12 +0900 From: Hyeonggon Yoo <42.hyeyoo@gmail.com> To: Matthew Wilcox Cc: linux-mm@kvack.org, liam.howlett@oracle.com, surenb@google.com, ldufour@linux.ibm.com, michel@lespinasse.org, vbabka@suse.cz, linux-kernel@vger.kernel.org Subject: Re: [QUESTION] about the maple tree and current status of mmap_lock scalability Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Hyeonggon Yoo <42.hyeyoo@gmail.com> To: Matthew Wilcox Cc: linux-mm@kvack.org, liam.howlett@oracle.com, surenb@google.com, ldufour@linux.ibm.com, michel@lespinasse.org, vbabka@suse.cz, linux-kernel@vger.kernel.org Bcc: Subject: Re: [QUESTION] about the maple tree and current status of mmap_lock scalability Reply-To: In-Reply-To: On Thu, Dec 29, 2022 at 04:51:37PM +0000, Matthew Wilcox wrote: > On Thu, Dec 29, 2022 at 11:22:28PM +0900, Hyeonggon Yoo wrote: > > On Wed, Dec 28, 2022 at 08:50:36PM +0000, Matthew Wilcox wrote: > > > The long term goal is even larger than this. Ideally, the VMA tree > > > would be protected by a spinlock rather than a mutex. > > > > You mean replacing mmap_lock rwsem with a spinlock? > > How is that possible if readers can take it for page fault? > > The mmap_lock is taken for many, many things. So the plan was to > have a spinlock in the maple tree (indeed, there's still one there; > it's just in a union with the lockdep_map_p). VMA readers would walk > the tree protected only by RCU; VMA writers would take the spinlock > while modifying the tree. The work Suren, Liam & I are engaged in > still uses the mmap semaphore for writers, but we do walk the tree > under RCU protection. > Thanks, I get it. so it's for less overhead for maple tree modification. > > > While I've read the RCUVM paper, I wouldn't say it was particularly an > > > inspiration. The Maple Tree is independent of the VM; it's a general > > > purpose B-tree. > > > > My intention was to ask how to synchronize with other VMA operations > > after the tree traversal with RCU. (Because it's unreasonable to handle > > page fault in RCU read-side critical section) > > > > Per-VMA lock seem to solve it by taking the VMA lock in read mode within > > RCU read-side critical section. > > Right, but it's a little more complex than that. The real "lock" on > the VMA is actually a sequence count. https://lwn.net/Articles/906852/ > does a good job of explaining it, but the VMA lock is really there as > a convenient way for the writer to wait for readers to be sufficiently > "finished" with handling the page fault that any conflicting changes > will be correctly retired. Oh, thanks, nice article! > https://www.infradead.org/~willy/linux/store-free-page-faults.html > outlines how I intend to proceed from Suren's current scheme (where > RCU is only used to protect the tree walk) to using RCU for the > entire page fault. Thank you for sharing this your outlines. Okay, so the planned scheme is: 1. Try to process entire page fault under RCU protection - if failed, goto 2. if succeeded, goto 4. 2. Fall back to Suren's scheme (try to take VMA rwsem) - if failed, goto 3. if succeeded, goto 4. 3. Fall back to mmap_lock - goto 4. 4. Finish page fault. To implement 1, __p*d_alloc() need to take gfp flags not to sleep in RCU read-side critical section. What about introducing PF_MEMALLOC_NOWAIT process flag forcing GFP_NOWAIT | __GFP_NOWARN similar to PF_MEMALLOC_NO{FS,IO}, looking like this? Will be less churn. diff --git a/include/linux/sched.h b/include/linux/sched.h index 853d08f7562b..77b88f30523b 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1725,7 +1725,7 @@ extern struct pid *cad_pid; #define PF_USED_MATH 0x00002000 /* If unset the fpu must be initialized before use */ #define PF__HOLE__00004000 0x00004000 #define PF_NOFREEZE 0x00008000 /* This thread should not be frozen */ -#define PF__HOLE__00010000 0x00010000 +#define PF_MEMALLOC_NOWAIT 0x00010000 /* All allocation requests will force GFP_NOWAIT | __GFP_NOWARN */ #define PF_KSWAPD 0x00020000 /* I am kswapd */ #define PF_MEMALLOC_NOFS 0x00040000 /* All allocation requests will inherit GFP_NOFS */ #define PF_MEMALLOC_NOIO 0x00080000 /* All allocation requests will inherit GFP_NOIO */ diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index 2a243616f222..4a1196646951 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -204,7 +204,8 @@ static inline gfp_t current_gfp_context(gfp_t flags) { unsigned int pflags = READ_ONCE(current->flags); - if (unlikely(pflags & (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_PIN))) { + if (unlikely(pflags & (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS + | PF_MEMALLOC_PIN | PF_MEMALLOC_NOWAIT))) { /* * NOIO implies both NOIO and NOFS and it is a weaker context * so always make sure it makes precedence @@ -216,6 +217,8 @@ static inline gfp_t current_gfp_context(gfp_t flags) if (pflags & PF_MEMALLOC_PIN) flags &= ~__GFP_MOVABLE; + if (pflags & PF_MEMALLOC_NOWAIT) + flags = GFP_NOWAIT | __GFP_NOWARN; } return flags; } @@ -305,6 +308,18 @@ static inline void memalloc_noio_restore(unsigned int flags) current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flags; } +static inline unsigned int memalloc_nowait_save(void) +{ + unsigned int flags = current->flags & PF_MEMALLOC_NOWAIT; + current->flags |= PF_MEMALLOC_NOWAIT; + return flags; +} + +static inline void memalloc_nowait_restore(unsigned int flags) +{ + current->flags = (current->flags & ~PF_MEMALLOC_NOWAIT) | flags; -- Thanks, Hyeonggon