Re: [PATCH v2] mm/compaction: remove unnecessary detection code.

[Vlastimil Babka]

On 11/14/24 07:57, Qiang Liu wrote:
> It is impossible for the situation where blockpfn > end_pfn to arise,
> The if statement here is not only unnecessary, but may also lead to
> a misunderstanding that blockpfn > end_pfn could potentially happen.
> so these unnecessary checking code should be removed.
> 
> Signed-off-by: Qiang Liu <liuq131@xxxxxxxxxxxxxxx>

I see that's since 3da0272a4c7d ("mm/compaction: correctly return failure
with bogus compound_order in strict mode")

I think that commit introduced a risk of overflow due to a bogus order
(which we read in a racy way), and once blockpfn overflows it will satisfy
<= end_pfn and might e.g. end up scanning a completely different zone?

                        if (blockpfn + (1UL << order) <= end_pfn) {

                                blockpfn += (1UL << order) - 1;
                                page += (1UL << order) - 1;
                                nr_scanned += (1UL << order) - 1;
                        }

We should better add back the MAX_ORDER sanity check?

> ---
>  mm/compaction.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index a2b16b08cbbf..baeda7132252 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -682,12 +682,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  	if (locked)
>  		spin_unlock_irqrestore(&cc->zone->lock, flags);
>  
> -	/*
> -	 * Be careful to not go outside of the pageblock.
> -	 */
> -	if (unlikely(blockpfn > end_pfn))
> -		blockpfn = end_pfn;
> -
>  	trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn,
>  					nr_scanned, total_isolated);
>