Re: page cache: Store only head pages in i_pages

[Qian Cai <cai@lca.pw>]

On Fri, 2019-03-29 at 12:59 -0700, Matthew Wilcox wrote:
> I don't understand how we get to this situation.  We SetPageSwapCache()
> in add_to_swap_cache() right before we store the page in i_pages.
> We ClearPageSwapCache() in __delete_from_swap_cache() right after
> removing the page from the array.  So how do we find a page in a swap
> address space that has PageSwapCache cleared?
> 
> Indeed, we have a check which should trigger ...
> 
>         VM_BUG_ON_PAGE(!PageSwapCache(page), page);
> 
> in __delete_from_swap_cache().
> 
> Oh ... is it a race?
> 
>  * Its ok to check for PageSwapCache without the page lock
>  * here because we are going to recheck again inside
>  * try_to_free_swap() _with_ the lock.
> 
> so CPU A does:
> 
> page = find_get_page(swap_address_space(entry), offset)
>         page = find_subpage(page, offset);
> trylock_page(page);
> 
> while CPU B does:
> 
> xa_lock_irq(&address_space->i_pages);
> __delete_from_swap_cache(page, entry);
>         xas_store(&xas, NULL);
>         ClearPageSwapCache(page);
> xa_unlock_irq(&address_space->i_pages);
> 
> and if the ClearPageSwapCache happens between the xas_load() and the
> find_subpage(), we're stuffed.  CPU A has a reference to the page, but
> not a lock, and find_get_page is running under RCU.
> 
> I suppose we could fix this by taking the i_pages xa_lock around the
> call to find_get_pages().  If indeed, that's what this problem is.
> Want to try this patch?

Confirmed. Well spotted!

> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2b8d9c3fbb47..ed8e42be88b5 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -127,10 +127,14 @@ static int __try_to_reclaim_swap(struct swap_info_struct
> *si,
>  				 unsigned long offset, unsigned long flags)
>  {
>  	swp_entry_t entry = swp_entry(si->type, offset);
> +	struct address_space *mapping = swap_address_space(entry);
> +	unsigned long irq_flags;
>  	struct page *page;
>  	int ret = 0;
>  
> -	page = find_get_page(swap_address_space(entry), offset);
> +	xa_lock_irqsave(&mapping->i_pages, irq_flags);
> +	page = find_get_page(mapping, offset);
> +	xa_unlock_irqrestore(&mapping->i_pages, irq_flags);
>  	if (!page)
>  		return 0;
>  	/*