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

[Vlastimil Babka <vbabka@suse.cz>]

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@kernel.org/
> Fixes: 7efc3b726103 ("mm/compaction: fix set skip in fast_find_migrateblock")
> Cc: <stable@vger.kernel.org>

Oops, forgot:

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  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;
>  			}
>  		}