Re: [PATCH v3 3/6] mm/compaction: correctly return failure with bogus compound_order in strict mode

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Sep 01, 2023 at 05:32:49PM +0800, Kemeng Shi wrote:
> 
> 
> 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!
> 

Ok, fair enough. While I think the check is redundant right now, it's a
reasonable defensive check and this is not a fast path so

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs