Re: [PATCH v5 2/5] mm,compaction: Let isolate_migratepages_{range,block} return error codes

[Oscar Salvador <osalvador@suse.de>]

On Wed, Mar 17, 2021 at 03:12:29PM +0100, Michal Hocko wrote:
> > 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.

When discussing this with Vlastimil, I came up with the idea of adding a new
field in compact_control struct, e.g: next_pfn_scan to keep track of the next
pfn to be scanned.

But Vlastimil made me realize that since cc->migrate_pfn points to that aleady,
so we do not need any extra field.

> > @@ -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.

When coding that, I thought about using -1 for the first two checks, and keep
-EINTR for the signal check, but isolate_migratepages_block only has two users:

- isolate_migratepages: Does not care about the return code other than pfn != 0,
  and it does not pass the error down the chain.
- isolate_migratepages_range: The error is passed down the chain, and !pfn is being
  treated as -EINTR:

static int __alloc_contig_migrate_range(struct compact_control *cc,
					unsigned long start, unsigned long end)
 {
  ...
  ...
  pfn = isolate_migratepages_range(cc, pfn, end);
  if (!pfn) {
          ret = -EINTR;
          break;
  }
  ...
 }

That is why I decided to stick with -EINTR.


-- 
Oscar Salvador
SUSE L3