Re: zswap z3fold + memory offline = infinite loop

[David Hildenbrand <david@redhat.com>]

On 13.05.20 02:36, Qian Cai wrote:
> Put zswap z3fold pages into the memory and then offline those memory would trigger an infinite loop here in
> 
> __offline_pages() --> do_migrate_range() because there is no error handling,

That's usually not the case, that's why we - nowadays - basically never
see endless loops anymore, except when some code is doing something
wrong with movable pages. The TODO below is somewhat misleading.

> 
> 			if (pfn) {
> 				/*
> 				 * TODO: fatal migration failures should bail
> 				 * out
> 				 */
> 				do_migrate_range(pfn, end_pfn);
> 
> There, isolate_movable_page() will always return -EBUSY  because,
> 
> 	if (!mapping->a_ops->isolate_page(page, mode))
> 		goto out_no_isolated;
> 
> i.e., z3fold_page_isolate() will always return false because,
> 
> zhdr->mapped_count == 2

Whenever we have such endless loops when offlining it's either because
1. Somebody puts unmovable data into the movable zone
2. Somebody marks unmovable data as movable
3. Somebody turns movable data into unmovable data although the
pageblock is isolated

After start_isolate_page_range() we are sure that we only have movable
data in the isolated page range. The pageblocks are isolated, so e.g.,
no new allocations can end up on that memory.

Offlining code can therefore assume that all memory is in fact movable
and can be moved eventually. It might not always work on the first
attempt, that's why we retry.

We don't expect permanent migration failure, because that would mean the
page is unmovable.

I can see that mm/z3fold.c uses  __SetPageMovable(). So the question is,
why are these pages permanently unmovable.


> 
> It should be easy to reproduce. Otherwise, use this one,
> 
> https://github.com/cailca/linux-mm/blob/master/random.c
> 
> and then watch the console burning with those,
> 
> [12661.793667][T566417] failed to isolate pfn 1045b2
> [12661.793745][T566417] page:c00c000004116c80 refcount:2 mapcount:0 mapping:00000000999f9672 index:0x0
> [12661.793865][T566417] mapping->a_ops:z3fold_aops
> [12661.793919][T566417] flags: 0x3fffc000000000()
> [12661.793969][T566417] raw: 003fffc000000000 c00c000003cef388 c00c000006b0da08 c000001275b87f6a
> [12661.794071][T566417] raw: 0000000000000000 0000000000000000 00000002ffffffff 0000000000000000
> [12661.794158][T566417] page dumped because: isolation failed
> [12661.794226][T566417] page_owner tracks the page as allocated
> [12661.794292][T566417] page last allocated via order 0, migratetype Unmovable, gfp_mask 0x12800(GFP_NOWAIT|__GFP_NOWARN|__GFP_NORETRY)
> [12661.794463][T566417]  prep_new_page+0x3d0/0x450
> [12661.794508][T566417]  get_page_from_freelist+0x1bb8/0x27c0
> [12661.794575][T566417]  __alloc_pages_slowpath.constprop.60+0x240/0x15a0
> [12661.794654][T566417]  __alloc_pages_nodemask+0x520/0x650
> [12661.794715][T566417]  alloc_pages_current+0xbc/0x140
> [12661.794772][T566417]  z3fold_zpool_malloc+0x6cc/0xe20
> [12661.794826][T566417]  zpool_malloc+0x34/0x50
> [12661.794888][T566417]  zswap_frontswap_store+0x60c/0xe20
> [12661.794942][T566417]  __frontswap_store+0x128/0x330
> [12661.794995][T566417]  swap_writepage+0x58/0x110
> [12661.795048][T566417]  pageout+0x16c/0xa40
> [12661.795092][T566417]  shrink_page_list+0x1ab4/0x2490
> [12661.795155][T566417]  shrink_inactive_list+0x25c/0x710
> [12661.795206][T566417]  shrink_lruvec+0x444/0x1260
> [12661.795274][T566417]  shrink_node+0x288/0x9a0
> [12661.795330][T566417]  do_try_to_free_pages+0x158/0x640
> [12661.795383][T566417] page last free stack trace:
> [12661.795437][T566417]  free_pcp_prepare+0x52c/0x590
> [12661.795493][T566417]  free_unref_page+0x38/0xf0
> [12662.156109][T566417]  free_z3fold_page+0x58/0x120
> [12662.156131][T566417]  free_pages_work+0x148/0x1c0
> [12662.156195][T566417]  process_one_work+0x310/0x900
> [12662.156257][T566417]  worker_thread+0x78/0x530
> [12662.156306][T566417]  kthread+0x1c4/0x1d0
> [12662.156354][T566417]  ret_from_kernel_thread+0x5c/0x74
> 


-- 
Thanks,

David / dhildenb