Messages in this thread | | | Date | Mon, 10 Aug 2009 12:16:44 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [BUGFIX][1/2] mm: add_to_swap_cache() must not sleep |
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On Mon, 10 Aug 2009 11:26:41 +0900 Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> 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. > yes.
> 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. > Hmm..
> add_to_swap() and shmem_writepage() call add_to_swap_cache() w/o __GFP_WAIT, > but read_swap_cache_async() can call it w/ __GFP_WAIT, so it can cause > soft lockup. > > This patch changes the gfp_mask of add_to_swap_cache() in read_swap_cache_async(). > > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Thank you for catching. Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
But Hm...I wonder whether this is the best fix.
If I was you, I may do following.
1. remove radix_tree_preload() and gfp_mask from add_to_swapcache(). Then, rename it fo __add_to_swapcache(). Or, move swap_duplicate() into add_to_swapcache() with a new flag.
2. do things in following order.
radix_tree_peload(); swap_duplicate(); # this never sleeps. add_to_swapcache() radix_tree_peload_end();
Good point of this approach is - we can use __GFP_WAIT in gfp_mask. - -ENOMEM means OOM, then, we should be aggressive to get a page.
How do you think ?
Thanks, -Kame
> --- > mm/swap_state.c | 3 ++- > 1 files changed, 2 insertions(+), 1 deletions(-) > > diff --git a/mm/swap_state.c b/mm/swap_state.c > index 42cd38e..3e6dd72 100644 > --- a/mm/swap_state.c > +++ b/mm/swap_state.c > @@ -76,6 +76,7 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask) > VM_BUG_ON(!PageLocked(page)); > VM_BUG_ON(PageSwapCache(page)); > VM_BUG_ON(!PageSwapBacked(page)); > + VM_BUG_ON(gfp_mask & __GFP_WAIT); > > error = radix_tree_preload(gfp_mask); > if (!error) { > @@ -307,7 +308,7 @@ 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, GFP_ATOMIC); > if (likely(!err)) { > /* > * Initiate read into locked page and return. >
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