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Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.14.0 Subject: Re: [PATCH 4/9] mm/compaction: simplify pfn iteration in isolate_freepages_range To: Kemeng Shi , 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: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: E7626140006 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: bpmqooa9tewpg3bssxqyq346ernfkdt5 X-HE-Tag: 1692844154-603780 X-HE-Meta: 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 lJumSu24 R6Ra29xEFBCrOG2MlxDwypeybjYSmAN/WwU59TV4sK+CE+is4BtO/CZ1d7jpw3qs+Yk/ornYBT+LqmmP7L3UN4uZ6nhzC0YKryrQMlcRmQJ0SYyjTNVpW118hcVRxfABNCmQ0dHsnP2IQ1t952PY5MfYlULYs86zfHwqgTtuT5NuUT5jxaRMjsmLgol6o6lACaCgOeKZqhHRIL4M= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 8/22/2023 9:37 AM, Kemeng Shi wrote: > > > on 8/19/2023 7:58 PM, Baolin Wang wrote: >> >> >> On 8/15/2023 6:37 PM, Kemeng Shi wrote: >>> >>> >>> on 8/15/2023 6:07 PM, Baolin Wang wrote: >>>> >>>> >>>> On 8/15/2023 5:32 PM, Kemeng Shi wrote: >>>>> >>>>> >>>>> on 8/15/2023 4:38 PM, Baolin Wang wrote: >>>>>> >>>>>> >>>>>> On 8/5/2023 7:07 PM, Kemeng Shi wrote: >>>>>>> We call isolate_freepages_block in strict mode, continuous pages in >>>>>>> pageblock will be isolated if isolate_freepages_block successed. >>>>>>> Then pfn + isolated will point to start of next pageblock to scan >>>>>>> no matter how many pageblocks are isolated in isolate_freepages_block. >>>>>>> Use pfn + isolated as start of next pageblock to scan to simplify the >>>>>>> iteration. >>>>>> >>>>>> IIUC, the isolate_freepages_block() can isolate high-order free pages, which means the pfn + isolated can be larger than the block_end_pfn. 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. I realized I made a bad example, sorry for noise. After more thinking, I agree that the 'pfn + isolated' is always pageblock aligned in strict mode. So feel free to add: Reviewed-by: Baolin Wang > 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, >>>>>> >>>> >> >>