linux-kernel - Re: [PATCH v3 3/6] mm/compaction: correctly return failure with bogus compound

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Date:   Fri, 1 Sep 2023 10:17:11 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Kemeng Shi <shikemeng@...weicloud.com>
Cc:     akpm@...ux-foundation.org, baolin.wang@...ux.alibaba.com,
        david@...hat.com, willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/6] mm/compaction: correctly return failure with
 bogus compound_order in strict mode

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@...weicloud.com>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.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?

-- 
Mel Gorman
SUSE Labs