Re: BUG: unable to handle kernel paging request in isolate_freepages_block

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, May 07, 2019 at 02:50:05AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    baf76f0c slip: make slhc_free() silently accept an error p..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=16dbe6cca00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a42d110b47dd6b36
> dashboard link: https://syzkaller.appspot.com/bug?extid=d84c80f9fe26a0f7a734
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> 
> Unfortunately, I don't have any reproducer for this crash yet.
> 

How reproducible is it and can the following (compile tested only) patch
be tested please? I'm thinking it's a similar class of bug to 6b0868c820ff
("mm/compaction.c: correct zone boundary handling when resetting pageblock
skip hints")

diff --git a/mm/compaction.c b/mm/compaction.c
index 3319e0872d01..ae4d99d31b61 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1228,7 +1228,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long
 
 	/* Pageblock boundaries */
 	start_pfn = pageblock_start_pfn(pfn);
-	end_pfn = min(start_pfn + pageblock_nr_pages, zone_end_pfn(cc->zone));
+	end_pfn = min(start_pfn + pageblock_nr_pages, zone_end_pfn(cc->zone) - 1);
 
 	/* Scan before */
 	if (start_pfn != pfn) {
@@ -1239,7 +1239,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long
 
 	/* Scan after */
 	start_pfn = pfn + nr_isolated;
-	if (start_pfn != end_pfn)
+	if (start_pfn < end_pfn)
 		isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, 1, false);
 
 	/* Skip this pageblock in the future as it's full or nearly full */