Testing shows fast_isolate_freepages can blindly choose an unsuitable
pageblock from time to time particularly while the min mark is used from
XXX path:

 if (!page) {
         cc->fast_search_fail++;
         if (scan_start) {
                 /*
                  * Use the highest PFN found above min. If one was
                  * not found, be pessimistic for direct compaction
                  * and use the min mark.
                  */
                 if (highest >= min_pfn) {
                         page = pfn_to_page(highest);
                         cc->free_pfn = highest;
                 } else {
                         if (cc->direct_compaction && pfn_valid(min_pfn)) { /* XXX */
                                 page = pageblock_pfn_to_page(min_pfn,
                                         min(pageblock_end_pfn(min_pfn),
                                             zone_end_pfn(cc->zone)),
                                         cc->zone);
                                 cc->free_pfn = min_pfn;
                         }
                 }
         }
 }

The reason is that no code is doing any check on the min_pfn
 min_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 1));

In contrast, slow path of isolate_freepages() is always skipping
unsuitable pageblocks in a decent way.

This issue doesn't happen quite often.  When running 25 machines with
16GiB memory for one night, most of them can hit this unexpected code
path.  However the frequency isn't like many times per second.  It might
be one time in a couple of hours.  Thus, it is very hard to measure the
visible performance impact in my machines though the affection of choosing
the unsuitable migration_target should be negative in theory.

I feel it's still worth fixing this to at least make the code
theoretically self-explanatory as it is quite odd an unsuitable
migration_target can be still migration_target.

Link: https://lkml.kernel.org/r/20231206110054.61617-1-v-songbaohua@oppo.comSigned-off-by: Barry Song <v-songbaohua@oppo.com>
Reported-by: Zhanyuan Hu <huzhanyuan@oppo.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>