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
