linux-kernel - Re: [PATCH v5 2/5] mm,compaction: Let isolate_migratepages

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Date:   Wed, 17 Mar 2021 15:12:29 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/5] mm,compaction: Let
 isolate_migratepages_{range,block} return error codes

On Wed 17-03-21 12:12:48, Oscar Salvador wrote:
> Currently, isolate_migratepages_{range,block} and their callers use
> a pfn == 0 vs pfn != 0 scheme to let the caller know whether there was
> any error during isolation.
> This does not work as soon as we need to start reporting different error
> codes and make sure we pass them down the chain, so they are properly
> interpreted by functions like e.g: alloc_contig_range.
> 
> Let us rework isolate_migratepages_{range,block} so we can report error
> codes.

Yes this is an improvement.

> Since isolate_migratepages_block will stop returning the next pfn to be
> scanned, we reuse the cc->migrate_pfn field to keep track of that.

This looks hakish and I cannot really tell that users of cc->migrate_pfn
work as intended.
> @@ -810,6 +811,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  	unsigned long next_skip_pfn = 0;
>  	bool skip_updated = false;
>  
> +	cc->migrate_pfn = low_pfn;
> +
>  	/*
>  	 * Ensure that there are not too many pages isolated from the LRU
>  	 * list by either parallel reclaimers or compaction. If there are,
> @@ -818,16 +821,16 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  	while (unlikely(too_many_isolated(pgdat))) {
>  		/* stop isolation if there are still pages not migrated */
>  		if (cc->nr_migratepages)
> -			return 0;
> +			return -EINTR;
>  
>  		/* async migration should just abort */
>  		if (cc->mode == MIGRATE_ASYNC)
> -			return 0;
> +			return -EINTR;

EINTR for anything other than signal based bail out is really confusing.
-- 
Michal Hocko
SUSE Labs