Re: [PATCH for 6.1 regression] Revert "mm/compaction: fix set skip in fast_find_migrateblock"

[Pedro Falcato]

On Sat, Jan 14, 2023 at 6:51 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 1/13/23 18:33, Vlastimil Babka wrote:
> > This reverts commit 7efc3b7261030da79001c00d92bc3392fd6c664c.
> >
> > We have got openSUSE reports (Link 1) for 6.1 kernel with khugepaged
> > stalling CPU for long periods of time. Investigation of tracepoint data
> > shows that compaction is stuck in repeating fast_find_migrateblock()
> > based migrate page isolation, and then fails to migrate all isolated
> > pages. Commit 7efc3b726103 ("mm/compaction: fix set skip in
> > fast_find_migrateblock") was suspected as it was merged in 6.1 and in
> > theory can indeed remove a termination condition for
> > fast_find_migrateblock() under certain conditions, as it removes a place
> > that always marks a scanned pageblock from being re-scanned. There are
> > other such places, but those can be skipped under certain conditions,
> > which seems to match the tracepoint data.
> >
> > Testing of revert also appears to have resolved the issue, thus revert
> > the commit until a more robust solution for the original problem is
> > developed.
> >
> > It's also likely this will fix qemu stalls with 6.1 kernel reported in
> > Link 2, but that is not yet confirmed.
> >
> > Link: https://bugzilla.suse.com/show_bug.cgi?id=1206848
> > Link: https://lore.kernel.org/kvm/b8017e09-f336-3035-8344-c549086c2340@xxxxxxxxxx/
> > Fixes: 7efc3b726103 ("mm/compaction: fix set skip in fast_find_migrateblock")
> > Cc: <stable@xxxxxxxxxxxxxxx>
>
> Oops, forgot:
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> > ---
> >  mm/compaction.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index ca1603524bbe..8238e83385a7 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -1839,6 +1839,7 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
> >                                       pfn = cc->zone->zone_start_pfn;
> >                               cc->fast_search_fail = 0;
> >                               found_block = true;
> > +                             set_pageblock_skip(freepage);
> >                               break;
> >                       }
> >               }
>

Vlastimil,

Thank you so much for looking into this. I've been daily driving it
for the past half day and it seems to have fixed my QEMU issues.
Of course, I don't have exactly a test suite for this but I've tried
everything and I can't get any of the original problems to show up.

That being said,
Tested-by: Pedro Falcato <pedro.falcato@xxxxxxxxx>

I'll report back if QEMU freezes the system again.

-- 
Pedro