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Subject: Re: [PATCH v3 3/6] mm/compaction: correctly return failure with bogus
 compound_order in strict mode
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     akpm@linux-foundation.org, baolin.wang@linux.alibaba.com,
        david@redhat.com, willy@infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20230901155141.249860-1-shikemeng@huaweicloud.com>
 <20230901155141.249860-4-shikemeng@huaweicloud.com>
 <20230901091711.rpykbvfsnrcuwqhn@techsingularity.net>
From:   Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <74851ecc-93ff-1ec3-1589-f13d66d4336d@huaweicloud.com>
Date:   Fri, 1 Sep 2023 17:32:49 +0800
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on 9/1/2023 5:17 PM, Mel Gorman wrote:
> On Fri, Sep 01, 2023 at 11:51:38PM +0800, Kemeng Shi wrote:
>> In strict mode, we should return 0 if there is any hole in pageblock. If
>> we successfully isolated pages at beginning at pageblock and then have a
>> bogus compound_order outside pageblock in next page. We will abort search
>> loop with blockpfn > end_pfn. Although we will limit blockpfn to end_pfn,
>> we will treat it as a successful isolation in strict mode as blockpfn is
>> not < end_pfn and return partial isolated pages. Then
>> isolate_freepages_range may success unexpectly with hole in isolated
>> range.
>>
>> Fixes: 9fcd6d2e052e ("mm, compaction: skip compound pages by order in free scanner")
>> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>  mm/compaction.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index a40550a33aee..9ecbfbc695e5 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -626,11 +626,12 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>>  		if (PageCompound(page)) {
>>  			const unsigned int order = compound_order(page);
>>  
>> -			if (likely(order <= MAX_ORDER)) {
>> +			if (blockpfn + (1UL << order) <= end_pfn) {
>>  				blockpfn += (1UL << order) - 1;
>>  				page += (1UL << order) - 1;
>>  				nr_scanned += (1UL << order) - 1;
>>  			}
>> +
>>  			goto isolate_fail;
>>  		}
>>  
>> @@ -678,8 +679,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>>  		spin_unlock_irqrestore(&cc->zone->lock, flags);
>>  
>>  	/*
>> -	 * There is a tiny chance that we have read bogus compound_order(),
>> -	 * so be careful to not go outside of the pageblock.
>> +	 * Be careful to not go outside of the pageblock.
>>  	 */
>>  	if (unlikely(blockpfn > end_pfn))
>>  		blockpfn = end_pfn;
> 
> Is this check still necessary after the first hunk?
> 
Actually, I removed this check in the first version, but Baolin thought remove this check is not
cheap and not worth it. More discussion can be found in [1]. Thanks!

[1] https://lore.kernel.org/all/a8edac8d-8e22-89cf-2c8c-217a54608d27@linux.alibaba.com/