On Wed, Jun 29, 2016 at 02:47:20PM -0700, David Rientjes wrote: > It's possible to isolate some freepages in a pageblock and then fail > split_free_page() due to the low watermark check. In this case, we hit > VM_BUG_ON() because the freeing scanner terminated early without a > contended lock or enough freepages. > > This should never have been a VM_BUG_ON() since it's not a fatal > condition. It should have been a VM_WARN_ON() at best, or even handled > gracefully. > > Regardless, we need to terminate anytime the full pageblock scan was not > done. The logic belongs in isolate_freepages_block(), so handle its state > gracefully by terminating the pageblock loop and making a note to restart > at the same pageblock next time since it was not possible to complete the > scan this time. > > Reported-by: Minchan Kim <[email protected]> > Signed-off-by: David Rientjes <[email protected]> > --- > Note: I really dislike the low watermark check in split_free_page() and > consider it poor software engineering. The function should split a free > page, nothing more. Terminating memory compaction because of a low > watermark check when we're simply trying to migrate memory seems like an > arbitrary heuristic. There was an objection to removing it in the first > proposed patch, but I think we should really consider removing that > check so this is simpler. > > mm/compaction.c | 37 +++++++++++++++---------------------- > 1 file changed, 15 insertions(+), 22 deletions(-) > > diff --git a/mm/compaction.c b/mm/compaction.c > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -1009,8 +1009,6 @@ static void isolate_freepages(struct compact_control > *cc) > block_end_pfn = block_start_pfn, > block_start_pfn -= pageblock_nr_pages, > isolate_start_pfn = block_start_pfn) { > - unsigned long isolated; > - > /* > * This can iterate a massively long zone without finding any > * suitable migration targets, so periodically check if we need > @@ -1034,36 +1032,31 @@ static void isolate_freepages(struct compact_control > *cc) > continue; > > /* Found a block suitable for isolating free pages from. */ > - isolated = isolate_freepages_block(cc, &isolate_start_pfn, > - block_end_pfn, freelist, false); > - /* If isolation failed early, do not continue needlessly */ > - if (!isolated && isolate_start_pfn < block_end_pfn && > - cc->nr_migratepages > cc->nr_freepages) > - break; > + isolate_freepages_block(cc, &isolate_start_pfn, block_end_pfn, > + freelist, false); > > /* > - * If we isolated enough freepages, or aborted due to async > - * compaction being contended, terminate the loop. > - * Remember where the free scanner should restart next time, > - * which is where isolate_freepages_block() left off. > - * But if it scanned the whole pageblock, isolate_start_pfn > - * now points at block_end_pfn, which is the start of the next > - * pageblock. > - * In that case we will however want to restart at the start > - * of the previous pageblock. > + * If we isolated enough freepages, or aborted due to lock > + * contention, terminate. > */ > if ((cc->nr_freepages >= cc->nr_migratepages) > || cc->contended) { > - if (isolate_start_pfn >= block_end_pfn) > + if (isolate_start_pfn >= block_end_pfn) { > + /* > + * Restart at previous pageblock if more > + * freepages can be isolated next time. > + */ > isolate_start_pfn = > block_start_pfn - pageblock_nr_pages; > + } > break; > - } else { > + } else if (isolate_start_pfn < block_end_pfn) { > /* > - * isolate_freepages_block() should not terminate > - * prematurely unless contended, or isolated enough > + * If isolation failed early, do not continue > + * needlessly. > */ > - VM_BUG_ON(isolate_start_pfn < block_end_pfn); > + isolate_start_pfn = block_start_pfn; > + break;
I don't think this line is correct. It would make cc->free_pfn go backward though it would not be a big problem. Just leaving isolate_start_pfn as isolate_freepages_block returns would be a proper solution here. Thanks.
