On Tue, Apr 17, 2012 at 08:52:21PM -0700, Hugh Dickins wrote: > > > (You do remind me that I meant years ago to switch swapper_space over > > > to the much simpler __set_page_dirty_no_writeback(), which shmem has > > > used for ages; but as far as this problem goes, that would probably > > > be at best a workaround, rather than the proper fix.) > > > > It would be a workaround. If in the future we wanted to treat swapper > > space more like a normal file inode and writeback dirty pages from > > the flusher thread then this bug would just pop its head back up. > > It's a no-brainer workaround: patch and more explanation below. I > can double-fix it if you prefer, but the one-liner appeals more to me. > Ok, fair enough. While I think swapper space will eventually use the dirty tag information that day is not today. > > > Hmm, mm/migrate.c. > > > > Migration moves the page mapping under the tree lock so > > __set_page_dirty_nobuffers() I don't think that is it. > > Yes, I was worried by the places that set page->mapping = NULL in > migrate.c (later, not under the tree_lock), but those would not be able > to generate this issue at all (ptes already replaced by migration entries). > Yes. > <SNIP> > > [PATCH] mm: fix s390 BUG by using __set_page_dirty_no_writeback on swap > > Mel reports a BUG_ON(slot == NULL) in radix_tree_tag_set() on s390 3.0.13: > called from __set_page_dirty_nobuffers() when page_remove_rmap() tries to > transfer dirty flag from s390 storage key to struct page and radix_tree. > > That would be because of reclaim's shrink_page_list() calling add_to_swap() > on this page at the same time: first PageSwapCache is set (causing > page_mapping(page) to appear as &swapper_space), then page->private set, > then tree_lock taken, then page inserted into radix_tree - so there's > an interval before taking the lock when the radix_tree slot is empty. > Yes, makes sense. > We could fix this by moving __add_to_swap_cache()'s spin_lock_irq up > before SetPageSwapCache, with error case ClearPageSwapCache moved up > under tree_lock too. > This can be done if/when swapper_space can make proper use of the dirty tag information. > But a better fix is just to do what's five years overdue. Ken Chen > added __set_page_dirty_no_writeback() (if !PageDirty TestSetPageDirty) > for tmpfs to skip all that radix_tree overhead, and swap is just the same: > it ignores the radix_tree tag, and does not participate in dirty page > accounting, so should be using __set_page_dirty_no_writeback() too. > > Reported-by: Mel Gorman <mgorman@xxxxxxx> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Acked-by: Mel Gorman <mgorman@xxxxxxx> I've sent a kernel based on this patch to the s390 folk that originally reported the bug. Hopefully they'll test and get back to me in a few days. Thanks Hugh. -- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
