On Sat, Dec 05, 2020 at 09:26:47PM -0500, Andrea Arcangeli wrote: > Hi Mel, > > On Thu, Nov 26, 2020 at 10:47:20AM +0000, Mel Gorman wrote: > > Agreed. This thread has a lot of different directions in it at this > > point so what I'd hope for is first, a patch that initialises holes with > > zone/node linkages within a 1<<(MAX_ORDER-1) alignment. If there is a > > hole, it would be expected the pages are PageReserved. Second, a fix to > > fast_isolate that forces PFNs returned to always be within the stated > > zone boundaries. > > The last two patches should resolve the struct page > initialization > https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/ and the > VM_BUG_ON_PAGE never happened again as expected. > > So I looked back to see how the "distance" logic is accurate. I added > those checks and I get negative hits: > > diff --git a/mm/compaction.c b/mm/compaction.c > index cc1a7f600a86..844a90b0acdf 100644 > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -1331,6 +1331,12 @@ fast_isolate_freepages(struct compact_control *cc) > low_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 2)); > min_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 1)); > > + WARN_ON_ONCE((long) low_pfn < 0); > + WARN_ON_ONCE((long) min_pfn < 0); > + if ((long) low_pfn < 0) > + return cc->free_pfn; > + if ((long) min_pfn < 0) > + return cc->free_pfn; > if (WARN_ON_ONCE(min_pfn > low_pfn)) > low_pfn = min_pfn; > > Both warn-on-once triggers, so it goes negative. While it appears not > kernel crashing since pfn_valid won't succeed on negative entries and > they'll always be higher than any pfn in the freelist, is this sign > that there's further room for improvement here? > Possibly, checking the wrong pfns is simply risky. This is not tested at all, just checking if it's in the right ballpark even. Intent is that when the free/migrate PFNs are too close or already overlapping that no attempt is made and it returns back to detect the scanners have met. diff --git a/mm/compaction.c b/mm/compaction.c index 13cb7a961b31..208cb5857446 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -1313,6 +1313,10 @@ fast_isolate_freepages(struct compact_control *cc) if (cc->order <= 0) return cc->free_pfn; + /* Ensure that migration and free scanner are not about to cross */ + if (cc->migrate_pfn < cc->free_pfn) + return cc->free_pfn; + /* * If starting the scan, use a deeper search and use the highest * PFN found if a suitable one is not found. @@ -1324,9 +1328,12 @@ fast_isolate_freepages(struct compact_control *cc) /* * Preferred point is in the top quarter of the scan space but take - * a pfn from the top half if the search is problematic. + * a pfn from the top half if the search is problematic. Ensure + * there is enough distance to make the fast search worthwhile. */ distance = (cc->free_pfn - cc->migrate_pfn); + if (distance <= (pageblock_nr_pages << 2)) + return cc->free_pfn; low_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 2)); min_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 1)); -- Mel Gorman SUSE Labs
