Re: [PATCH] mm: be more verbose for alloc_contig_range faliures

[Minchan Kim <minchan@kernel.org>]

On Thu, Mar 04, 2021 at 10:11:35AM -0800, Minchan Kim wrote:
> On Thu, Mar 04, 2021 at 06:23:09PM +0100, David Hildenbrand wrote:
> > > > You want to debug something, so you try triggering it and capturing debug
> > > > data. There are not that many alloc_contig_range() users such that this
> > > > would really be an issue to isolate ...
> > > 
> > > cma_alloc uses alloc_contig_range and cma_alloc has lots of users.
> > > Even, it is expoerted by dmabuf so any userspace would trigger the
> > > allocation by their own. Some of them could be tolerant for the failure,
> > > rest of them could be critical. We should't expect it by limited kernel
> > > usecase.
> > 
> > Assume you are debugging allocation failures. You either collect the data
> > yourself or ask someone to send you that output. You care about any
> > alloc_contig_range() allocation failures that shouldn't happen, don't you?
> > 
> > > 
> > > > 
> > > > Strictly speaking: any allocation failure on ZONE_MOVABLE or CMA is
> > > > problematic (putting aside NORETRY logic and similar aside). So any such
> > > > page you hit is worth investigating and, therefore, worth getting logged for
> > > > debugging purposes.
> > > 
> > > If you believe the every alloc_contig_range failure is problematic
> > 
> > Every one where we should have guarantees I guess: ZONE_MOVABLE or
> > MIGRAT_CMA. On ZONE_NORMAL, there are no guarantees.
> 
> Indeed.

How about this?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 238d0fc232aa..489e557b9390 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8481,7 +8481,8 @@ static inline void dump_migrate_failure_pages(struct list_head *page_list)

 /* [start, end) must belong to a single zone. */
 static int __alloc_contig_migrate_range(struct compact_control *cc,
-                                       unsigned long start, unsigned long end)
+                                       unsigned long start, unsigned long end,
+                                       bool nofail)
 {
        /* This function is based on compact_zone() from compaction.c. */
        unsigned int nr_reclaimed;
@@ -8522,7 +8523,8 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
                                NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
        }
        if (ret < 0) {
-               dump_migrate_failure_pages(&cc->migratepages);
+               if (ret == -EBUSY && nofail)
+                       dump_migrate_failure_pages(&cc->migratepages);
                putback_movable_pages(&cc->migratepages);
                return ret;
        }
@@ -8610,7 +8612,9 @@ int alloc_contig_range(unsigned long start, unsigned long end,
         * allocated.  So, if we fall through be sure to clear ret so that
         * -EBUSY is not accidentally used or returned to caller.
         */
-       ret = __alloc_contig_migrate_range(&cc, start, end);
+       ret = __alloc_contig_migrate_range(&cc, start, end,
+                                       migratetype == CMA ||
+                                       zone_idx(cc.zone) == ZONE_MOVABLE);
        if (ret && ret != -EBUSY)
                goto done;
        ret =0;