linux-kernel - Re: kernel BUG at mm/swap

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Date:   Thu, 25 Jul 2019 04:44:08 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>,
        huang ying <huang.ying.caritas@...il.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org
Subject: Re: kernel BUG at mm/swap_state.c:170!

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?