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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 21126EE49AA for ; Tue, 22 Aug 2023 01:37:19 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 7F14194001D; Mon, 21 Aug 2023 21:37:18 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 7A12B940008; Mon, 21 Aug 2023 21:37:18 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 668BC94001D; Mon, 21 Aug 2023 21:37:18 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 57384940008 for ; Mon, 21 Aug 2023 21:37:18 -0400 (EDT) Received: from smtpin06.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 95215160175 for ; 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s=arc-20220608; d=hostedemail.com; t=1692668235; a=rsa-sha256; cv=none; b=YKkHcM+yG5f7bMZ5dAeNGvQu3P/9EEj8SHEInVJvhKn2Lz5aYHrSm+b4TU+iF6stvY2jy4 fvnMwC2HieYVZsr20yAxIihYzwqRXzQMe2rkiI18vBEZC/wsewc6Ku87ZsbSIaD0A9FFEV NIJyACf47f5uLI1w6YzYr14Bdp9msks= ARC-Authentication-Results: i=1; imf02.hostedemail.com; dkim=none; spf=none (imf02.hostedemail.com: domain of shikemeng@huaweicloud.com has no SPF policy when checking 45.249.212.51) smtp.mailfrom=shikemeng@huaweicloud.com; dmarc=none Received: from mail02.huawei.com (unknown [172.30.67.153]) by dggsgout11.his.huawei.com (SkyGuard) with ESMTP id 4RVBm465kZz4f3kWY for ; Tue, 22 Aug 2023 09:37:08 +0800 (CST) Received: from [10.174.178.129] (unknown [10.174.178.129]) by APP3 (Coremail) with SMTP id _Ch0CgD3Fr9EEeRkvV7UBA--.32703S2; Tue, 22 Aug 2023 09:37:09 +0800 (CST) Subject: Re: [PATCH 4/9] mm/compaction: simplify pfn iteration in isolate_freepages_range To: Baolin Wang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, mgorman@techsingularity.net, david@redhat.com References: <20230805110711.2975149-1-shikemeng@huaweicloud.com> <20230805110711.2975149-5-shikemeng@huaweicloud.com> <43b726c1-3ea6-9acc-d4e4-c7deabcf7ecd@huaweicloud.com> <3729c50f-6f8e-2548-8932-f39045402299@linux.alibaba.com> <3574ed6e-34c8-47a1-8218-9e4cf1327184@huaweicloud.com> From: Kemeng Shi Message-ID: Date: Tue, 22 Aug 2023 09:37:08 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; 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So in your patch, the 'block_start_pfn' and 'block_end_pfn' can be in different pageblocks, that will break pageblock_pfn_to_page(). >>>>> >>>> In for update statement, we always update block_start_pfn to pfn and >>> >>> I mean, you changed to: >>> 1) pfn += isolated; >>> 2) block_start_pfn = pfn; >>> 3) block_end_pfn = pfn + pageblock_nr_pages; >>> >>> But in 1) pfn + isolated can go outside of the currnet pageblock if isolating a high-order page, for example, located in the middle of the next pageblock. So that the block_start_pfn can point to the middle of the next pageblock, not the start position. Meanwhile after 3), the block_end_pfn can point another pageblock. Or I missed something else? >>> >> Ah, I miss to explain this in changelog. >> In case we could we have buddy page with order higher than pageblock: >> 1. page in buddy page is aligned with it's order >> 2. order of page is higher than pageblock order >> Then page is aligned with pageblock order. So pfn of page and isolated pages >> count are both aligned pageblock order. So pfn + isolated is pageblock order >> aligned. > > That's not what I mean. pfn + isolated is not always pageblock-aligned, since the isolate_freepages_block() can isolated high-order free pages (for example: order-1, order-2 ...). > > Suppose the pageblock size is 2M, when isolating a pageblock (suppose the pfn range is 0 - 511 to make the arithmetic easy) by isolate_freepages_block(), and suppose pfn 0 to pfn 510 are all order-0 page, but pfn 511 is order-1 page, so you will isolate 513 pages from this pageblock, which will make 'pfn + isolated' not pageblock aligned. This is also no supposed to happen as low order buddy pages should never span cross boundary of high order pages: In buddy system, we always split order N pages into two order N - 1 pages as following: | order N | |order N - 1|order N - 1| So buddy pages with order N - 1 will never cross boudary of order N. Similar, buddy pages with order N - 2 will never cross boudary of order N - 1 and so on. Then any pages with order less than N will never cross boudary of order N. > >>>> update block_end_pfn to pfn + pageblock_nr_pages. So they should point >>>> to the same pageblock. I guess you missed the change to update of >>>> block_end_pfn. :) >>>>>> >>>>>> Signed-off-by: Kemeng Shi >>>>>> --- >>>>>>     mm/compaction.c | 14 ++------------ >>>>>>     1 file changed, 2 insertions(+), 12 deletions(-) >>>>>> >>>>>> diff --git a/mm/compaction.c b/mm/compaction.c >>>>>> index 684f6e6cd8bc..8d7d38073d30 100644 >>>>>> --- a/mm/compaction.c >>>>>> +++ b/mm/compaction.c >>>>>> @@ -733,21 +733,11 @@ isolate_freepages_range(struct compact_control *cc, >>>>>>         block_end_pfn = pageblock_end_pfn(pfn); >>>>>>           for (; pfn < end_pfn; pfn += isolated, >>>>>> -                block_start_pfn = block_end_pfn, >>>>>> -                block_end_pfn += pageblock_nr_pages) { >>>>>> +                block_start_pfn = pfn, >>>>>> +                block_end_pfn = pfn + pageblock_nr_pages) { >>>>>>             /* Protect pfn from changing by isolate_freepages_block */ >>>>>>             unsigned long isolate_start_pfn = pfn; >>>>>>     -        /* >>>>>> -         * pfn could pass the block_end_pfn if isolated freepage >>>>>> -         * is more than pageblock order. In this case, we adjust >>>>>> -         * scanning range to right one. >>>>>> -         */ >>>>>> -        if (pfn >= block_end_pfn) { >>>>>> -            block_start_pfn = pageblock_start_pfn(pfn); >>>>>> -            block_end_pfn = pageblock_end_pfn(pfn); >>>>>> -        } >>>>>> - >>>>>>             block_end_pfn = min(block_end_pfn, end_pfn); >>>>>>               if (!pageblock_pfn_to_page(block_start_pfn, >>>>> >>> > >