On Wed, Dec 13, 2023 at 9:02 PM Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> 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>
Hi Mel,
Thanks!
Hi Andrew,
Given this patch has been in mm-stable,
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?h=mm-stable&id=d19b1a1797
does it still have a chance to collect Mel's tag？
>
> --
> Mel Gorman
> SUSE Labs
Thanks
Barry
