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 X-Spam-Level: X-Spam-Status: No, score=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 97AB8C433E0 for ; Mon, 11 Jan 2021 21:39:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3800522287 for ; Mon, 11 Jan 2021 21:39:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731402AbhAKVjU (ORCPT ); Mon, 11 Jan 2021 16:39:20 -0500 Received: from relay.sw.ru ([185.231.240.75]:53800 "EHLO relay3.sw.ru" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727668AbhAKVjT (ORCPT ); Mon, 11 Jan 2021 16:39:19 -0500 Received: from [192.168.15.62] by relay3.sw.ru with esmtp (Exim 4.94) (envelope-from ) id 1kz4sM-00GEoi-P1; Tue, 12 Jan 2021 00:37:22 +0300 Subject: Re: [v3 PATCH 05/11] mm: vmscan: use a new flag to indicate shrinker is registered To: Yang Shi Cc: Roman Gushchin , Shakeel Butt , Dave Chinner , Johannes Weiner , Michal Hocko , Andrew Morton , Linux MM , Linux FS-devel Mailing List , Linux Kernel Mailing List References: <20210105225817.1036378-1-shy828301@gmail.com> <20210105225817.1036378-6-shy828301@gmail.com> From: Kirill Tkhai Message-ID: Date: Tue, 12 Jan 2021 00:37:23 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11.01.2021 21:17, Yang Shi wrote: > On Wed, Jan 6, 2021 at 2:22 AM Kirill Tkhai wrote: >> >> On 06.01.2021 01:58, Yang Shi wrote: >>> Currently registered shrinker is indicated by non-NULL shrinker->nr_deferred. >>> This approach is fine with nr_deferred at the shrinker level, but the following >>> patches will move MEMCG_AWARE shrinkers' nr_deferred to memcg level, so their >>> shrinker->nr_deferred would always be NULL. This would prevent the shrinkers >>> from unregistering correctly. >>> >>> Signed-off-by: Yang Shi >>> --- >>> include/linux/shrinker.h | 7 ++++--- >>> mm/vmscan.c | 13 +++++++++---- >>> 2 files changed, 13 insertions(+), 7 deletions(-) >>> >>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h >>> index 0f80123650e2..1eac79ce57d4 100644 >>> --- a/include/linux/shrinker.h >>> +++ b/include/linux/shrinker.h >>> @@ -79,13 +79,14 @@ struct shrinker { >>> #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */ >>> >>> /* Flags */ >>> -#define SHRINKER_NUMA_AWARE (1 << 0) >>> -#define SHRINKER_MEMCG_AWARE (1 << 1) >>> +#define SHRINKER_REGISTERED (1 << 0) >>> +#define SHRINKER_NUMA_AWARE (1 << 1) >>> +#define SHRINKER_MEMCG_AWARE (1 << 2) >>> /* >>> * It just makes sense when the shrinker is also MEMCG_AWARE for now, >>> * non-MEMCG_AWARE shrinker should not have this flag set. >>> */ >>> -#define SHRINKER_NONSLAB (1 << 2) >>> +#define SHRINKER_NONSLAB (1 << 3) >>> >>> extern int prealloc_shrinker(struct shrinker *shrinker); >>> extern void register_shrinker_prepared(struct shrinker *shrinker); >>> diff --git a/mm/vmscan.c b/mm/vmscan.c >>> index 8da765a85569..9761c7c27412 100644 >>> --- a/mm/vmscan.c >>> +++ b/mm/vmscan.c >>> @@ -494,6 +494,7 @@ void register_shrinker_prepared(struct shrinker *shrinker) >>> if (shrinker->flags & SHRINKER_MEMCG_AWARE) >>> idr_replace(&shrinker_idr, shrinker, shrinker->id); >>> #endif >>> + shrinker->flags |= SHRINKER_REGISTERED; >> >> In case of we introduce this new flag, we should kill old flag SHRINKER_REGISTERING, >> which are not needed anymore (we should you the new flag instead of that). > > The only think that I'm confused with is the check in > shrink_slab_memcg, it does: > > shrinker = idr_find(&shrinker_idr, i); > if (unlikely(!shrinker || shrinker == SHRINKER_REGISTERING)) { > > When allocating idr, the shrinker is associated with > SHRINKER_REGISTERING. But, shrink_slab_memcg does acquire read > shrinker_rwsem, and idr_alloc is called with holding write > shrinker_rwsem, so I'm supposed shrink_slab_memcg should never see > shrinker is registering. After prealloc_shrinker() shrinker is visible for shrink_slab_memcg(). This is the moment shrink_slab_memcg() sees SHRINKER_REGISTERED. > If so it seems easy to remove > SHRINKER_REGISTERING. > > We just need change that check to: > !shrinker || !(shrinker->flags & SHRINKER_REGISTERED) > >>> up_write(&shrinker_rwsem); >>> } >>> >>> @@ -513,13 +514,17 @@ EXPORT_SYMBOL(register_shrinker); >>> */ >>> void unregister_shrinker(struct shrinker *shrinker) >>> { >>> - if (!shrinker->nr_deferred) >>> - return; >>> - if (shrinker->flags & SHRINKER_MEMCG_AWARE) >>> - unregister_memcg_shrinker(shrinker); >>> down_write(&shrinker_rwsem); >> >> I do not think there are some users which registration may race with unregistration. >> So, I think we should check SHRINKER_REGISTERED unlocked similar to we used to check >> shrinker->nr_deferred unlocked. > > Yes, I agree. > >> >>> + if (!(shrinker->flags & SHRINKER_REGISTERED)) { >>> + up_write(&shrinker_rwsem); >>> + return; >>> + } >>> list_del(&shrinker->list); >>> + shrinker->flags &= ~SHRINKER_REGISTERED; >>> up_write(&shrinker_rwsem); >>> + >>> + if (shrinker->flags & SHRINKER_MEMCG_AWARE) >>> + unregister_memcg_shrinker(shrinker); >>> kfree(shrinker->nr_deferred); >>> shrinker->nr_deferred = NULL; >>> } >>> >> >>