Re: kernel BUG at mm/swap_state.c:170!

[Huang\, Ying]

Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:

> On Tue, Jul 23, 2019 at 01:08:42PM +0800, Huang, Ying wrote:
>> @@ -2489,6 +2491,14 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>  	/* complete memcg works before add pages to LRU */
>>  	mem_cgroup_split_huge_fixup(head);
>>  
>> +	if (PageAnon(head) && PageSwapCache(head)) {
>> +		swp_entry_t entry = { .val = page_private(head) };
>> +
>> +		offset = swp_offset(entry);
>> +		swap_cache = swap_address_space(entry);
>> +		xa_lock(&swap_cache->i_pages);
>> +	}
>> +
>>  	for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
>>  		__split_huge_page_tail(head, i, lruvec, list);
>>  		/* Some pages can be beyond i_size: drop them from page cache */
>> @@ -2501,6 +2511,9 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>  		} else if (!PageAnon(page)) {
>>  			__xa_store(&head->mapping->i_pages, head[i].index,
>>  					head + i, 0);
>> +		} else if (swap_cache) {
>> +			__xa_store(&swap_cache->i_pages, offset + i,
>> +				   head + i, 0);
>
> I tried something along these lines (though I think I messed up the offset
> calculation which is why it wasn't working for me).  My other concern
> was with the case where SWAPFILE_CLUSTER was less than HPAGE_PMD_NR.
> Don't we need to drop the lock and look up a new swap_cache if offset >=
> SWAPFILE_CLUSTER?

In swapfile.c, there is

#ifdef CONFIG_THP_SWAP
#define SWAPFILE_CLUSTER	HPAGE_PMD_NR
...
#else
#define SWAPFILE_CLUSTER	256
...
#endif

So, if a THP is in swap cache, then SWAPFILE_CLUSTER equals
HPAGE_PMD_NR.


And there is one swap address space for each 64M swap space.  So one THP
will be in one swap address space.

In swap.h, there is

/* One swap address space for each 64M swap space */
#define SWAP_ADDRESS_SPACE_SHIFT	14
#define SWAP_ADDRESS_SPACE_PAGES	(1 << SWAP_ADDRESS_SPACE_SHIFT)

Best Regards,
Huang, Ying