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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 51601C433E0 for ; Wed, 17 Mar 2021 14:24:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0FEED64F26 for ; Wed, 17 Mar 2021 14:24:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231643AbhCQOXb (ORCPT ); Wed, 17 Mar 2021 10:23:31 -0400 Received: from mx2.suse.de ([195.135.220.15]:41398 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231562AbhCQOXA (ORCPT ); Wed, 17 Mar 2021 10:23:00 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1615990979; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=hJZJQ8McgcLqhmV4lTLgUeWAHRAD2n58aQMbQhzih7I=; b=vZMNbbh1KOl1d2kF5e6qI+Hpi0dlwt6xMCTnXY3I3O4t8Un8MgyRX4x04t6oHG5M5gnO0d diSWHvsShphqGmaqKQLqcq2zmRc/lI0/rIbiTd6EWhoERtjkeXJn7xFE5PI6cOpTVjUenP yTBMtRJFhHYUHrT2s8qxcQgyGL4e9HU= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 22BC0AD72; Wed, 17 Mar 2021 14:22:59 +0000 (UTC) Date: Wed, 17 Mar 2021 15:22:58 +0100 From: Michal Hocko To: Oscar Salvador Cc: Andrew Morton , Vlastimil Babka , David Hildenbrand , Muchun Song , Mike Kravetz , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 3/5] mm: Make alloc_contig_range handle free hugetlb pages Message-ID: References: <20210317111251.17808-1-osalvador@suse.de> <20210317111251.17808-4-osalvador@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210317111251.17808-4-osalvador@suse.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 17-03-21 12:12:49, Oscar Salvador wrote: > alloc_contig_range will fail if it ever sees a HugeTLB page within the > range we are trying to allocate, even when that page is free and can be > easily reallocated. > This has proved to be problematic for some users of alloc_contic_range, > e.g: CMA and virtio-mem, where those would fail the call even when those > pages lay in ZONE_MOVABLE and are free. > > We can do better by trying to replace such page. > > Free hugepages are tricky to handle so as to no userspace application > notices disruption, we need to replace the current free hugepage with > a new one. > > In order to do that, a new function called alloc_and_dissolve_huge_page > is introduced. > This function will first try to get a new fresh hugepage, and if it > succeeds, it will replace the old one in the free hugepage pool. > > All operations are being handled under hugetlb_lock, so no races are Slightly confusing because allocation which is a part of the process is certainly not done under the lock. "The free page replacement is done under hugetlb_lock, so no external user of hugetlb will notice the change. There is one tricky case when page's refcount is 0 because it is in the process of being released. A mising PageHugeFreed bit will tell us that freeing is in flight so we retry after dropping the hugetlb_lock. The race window should be small and the next retry should make a forward progress. > possible. The only exception is when page's refcount is 0, but it still > has not been flagged as PageHugeFreed. > E.g, below scenario: > > CPU0 CPU1 > __free_huge_page() isolate_or_dissolve_huge_page > PageHuge() == T > alloc_and_dissolve_huge_page > alloc_fresh_huge_page() > spin_lock(hugetlb_lock) > // PageHuge() && !PageHugeFreed && > // !PageCount() > spin_unlock(hugetlb_lock) > spin_lock(hugetlb_lock) > 1) update_and_free_page > PageHuge() == F > __free_pages() > 2) enqueue_huge_page > SetPageHugeFreed() > spin_unlock(&hugetlb_lock) > spin_lock(hugetlb_lock) > 1) PageHuge() == F (freed by case#1 from CPU0) > 2) PageHuge() == T > PageHugeFreed() == T > - proceed with replacing the page > > In the case above we retry as the window race is quite small and we have high > chances to succeed next time. > > With regard to the allocation, we restrict it to the node the page belongs > to with __GFP_THISNODE, meaning we do not fallback on other node's zones. > > Note that gigantic hugetlb pages are fenced off since there is a cyclic > dependency between them and alloc_contig_range. > > Signed-off-by: Oscar Salvador > Reviewed-by: Mike Kravetz > Acked-by: Michal Hocko my ack still applies. -- Michal Hocko SUSE Labs