Re: [PATCH] mm, compaction: fix fast_isolate_around() to stay within boundaries

[Mel Gorman]

On Wed, Nov 09, 2022 at 05:41:12AM +0000, Akira Naribayashi (Fujitsu) wrote:
> On Mon, 7 Nov 2022 15:43:56 +0000, Mei Gorman wrote:
> > On Mon, Nov 07, 2022 at 12:32:34PM +0000, Akira Naribayashi (Fujitsu) wrote:
> > > > Under what circumstances will this panic occur?  I assume those
> > > > circumstnces are pretty rare, give that 6e2b7044c1992 was nearly two
> > > > years ago.
> > > > 
> > > > Did you consider the desirability of backporting this fix into earlier
> > > > kernels?
> > > 
> > > 
> > > Panic can occur on systems with multiple zones in a single pageblock.
> > > 
> > 
> > Please provide an example of the panic and the zoneinfo.
> 
> This issue is occurring in our customer's environment and cannot 
> be shared publicly as it contains customer information.
> Also, the panic is occurring with the kernel in RHEL and may not 
> panic with Upstream's community kernel.
> In other words, it is possible to panic on older kernels.
> I think this fix should be backported to stable kernel series.
> 
> > > The reason it is rare is that it only happens in special configurations.
> > 
> > How is this special configuration created?
> 
> This is the case when the node boundary is not aligned to pageblock boundary.

In that case, does this work to avoid rescanning an area that was already
isolated?

diff --git a/mm/compaction.c b/mm/compaction.c
index c51f7f545afe..58cf73ff20ff 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1346,7 +1346,7 @@ move_freelist_tail(struct list_head *freelist, struct page *freepage)
 static void
 fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long nr_isolated)
 {
-	unsigned long start_pfn, end_pfn;
+	unsigned long start_pfn, end_pfn, isolated_end;
 	struct page *page;
 
 	/* Do not search around if there are enough pages already */
@@ -1361,6 +1361,10 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long
 	start_pfn = max(pageblock_start_pfn(pfn), cc->zone->zone_start_pfn);
 	end_pfn = min(pageblock_end_pfn(pfn), zone_end_pfn(cc->zone));
 
+	/* Pageblock may straddle zone/node boundaries */
+	isolated_end = pfn + nr_isolated;
+	pfn = clamp(pfn, start_pfn, end_pfn);
+
 	page = pageblock_pfn_to_page(start_pfn, end_pfn, cc->zone);
 	if (!page)
 		return;
@@ -1373,7 +1377,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long
 	}
 
 	/* Scan after */
-	start_pfn = pfn + nr_isolated;
+	start_pfn = isolated_end;
 	if (start_pfn < end_pfn)
 		isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, 1, false);