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SubjectRe: [BUGFIX][1/2] mm: add_to_swap_cache() must not sleep
On Mon, 10 Aug 2009 14:49:41 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> Thank you for your suggestion. It's a good idea.
>
> How about this one (Passed build test only) ?
>
> If it's good for you, I'll resend it removing RFC after a long term test,
> unless someone else tell me not to.
>

Seems very nice to me. I hope answers from swap-people is good..

Thanks,
-Kame

> ===
> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
>
> After commit 355cfa73(mm: modify swap_map and add SWAP_HAS_CACHE flag),
> read_swap_cache_async() will busy-wait while a entry doesn't on swap cache
> but it has SWAP_HAS_CACHE flag.
>
> Such entries can exist on add/delete path of swap cache.
> On add path, add_to_swap_cache() is called soon after SWAP_HAS_CACHE flag
> is set, and on delete path, swapcache_free() will be called (SWAP_HAS_CACHE
> flag is cleared) soon after __delete_from_swap_cache() is called.
> So, the busy-wait works well in most cases.
>
> But this mechanism can cause soft lockup if add_to_swap_cache() sleeps
> and read_swap_cache_async() tries to swap-in the same entry on the same cpu.
>
> This patch calls radix_tree_preload() before swapcache_prepare() and divide
> add_to_swap_cache() into two part: radix_tree_preload() part and
> radix_tree_insert() part(define it as __add_to_swap_cache()).
>
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> ---
> mm/swap_state.c | 70 ++++++++++++++++++++++++++++++++++++-------------------
> 1 files changed, 46 insertions(+), 24 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 42cd38e..0313a13 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -66,10 +66,10 @@ void show_swap_cache_info(void)
> }
>
> /*
> - * add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
> + * __add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
> * but sets SwapCache flag and private instead of mapping and index.
> */
> -int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
> +static int __add_to_swap_cache(struct page *page, swp_entry_t entry)
> {
> int error;
>
> @@ -77,28 +77,37 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
> VM_BUG_ON(PageSwapCache(page));
> VM_BUG_ON(!PageSwapBacked(page));
>
> + page_cache_get(page);
> + SetPageSwapCache(page);
> + set_page_private(page, entry.val);
> +
> + spin_lock_irq(&swapper_space.tree_lock);
> + error = radix_tree_insert(&swapper_space.page_tree, entry.val, page);
> + if (likely(!error)) {
> + total_swapcache_pages++;
> + __inc_zone_page_state(page, NR_FILE_PAGES);
> + INC_CACHE_INFO(add_total);
> + }
> + spin_unlock_irq(&swapper_space.tree_lock);
> +
> + if (unlikely(error)) {
> + set_page_private(page, 0UL);
> + ClearPageSwapCache(page);
> + page_cache_release(page);
> + }
> +
> + return error;
> +}
> +
> +
> +int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
> +{
> + int error;
> +
> error = radix_tree_preload(gfp_mask);
> if (!error) {
> - page_cache_get(page);
> - SetPageSwapCache(page);
> - set_page_private(page, entry.val);
> -
> - spin_lock_irq(&swapper_space.tree_lock);
> - error = radix_tree_insert(&swapper_space.page_tree,
> - entry.val, page);
> - if (likely(!error)) {
> - total_swapcache_pages++;
> - __inc_zone_page_state(page, NR_FILE_PAGES);
> - INC_CACHE_INFO(add_total);
> - }
> - spin_unlock_irq(&swapper_space.tree_lock);
> + error = __add_to_swap_cache(page, entry);
> radix_tree_preload_end();
> -
> - if (unlikely(error)) {
> - set_page_private(page, 0UL);
> - ClearPageSwapCache(page);
> - page_cache_release(page);
> - }
> }
> return error;
> }
> @@ -289,13 +298,24 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> }
>
> /*
> + * call radix_tree_preload() while we can wait.
> + */
> + err = radix_tree_preload(gfp_mask & GFP_KERNEL);
> + if (err)
> + break;
> +
> + /*
> * Swap entry may have been freed since our caller observed it.
> */
> err = swapcache_prepare(entry);
> - if (err == -EEXIST) /* seems racy */
> + if (err == -EEXIST) { /* seems racy */
> + radix_tree_preload_end();
> continue;
> - if (err) /* swp entry is obsolete ? */
> + }
> + if (err) { /* swp entry is obsolete ? */
> + radix_tree_preload_end();
> break;
> + }
>
> /*
> * Associate the page with swap entry in the swap cache.
> @@ -307,8 +327,9 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> */
> __set_page_locked(new_page);
> SetPageSwapBacked(new_page);
> - err = add_to_swap_cache(new_page, entry, gfp_mask & GFP_KERNEL);
> + err = __add_to_swap_cache(new_page, entry);
> if (likely(!err)) {
> + radix_tree_preload_end();
> /*
> * Initiate read into locked page and return.
> */
> @@ -316,6 +337,7 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> swap_readpage(new_page);
> return new_page;
> }
> + radix_tree_preload_end();
> ClearPageSwapBacked(new_page);
> __clear_page_locked(new_page);
> swapcache_free(entry, NULL);
>



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