On Thu, Dec 07, 2023 at 12:00:54AM +1300, Barry Song wrote:
> 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.
>
> Reported-by: Zhanyuan Hu <huzhanyuan@xxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
--
Mel Gorman
SUSE Labs
