The patch titled Subject: dax: fix deadlock with DAX 4k holes has been added to the -mm tree. Its filename is dax-fix-deadlock-with-dax-4k-holes.patch This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/dax-fix-deadlock-with-dax-4k-holes.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/dax-fix-deadlock-with-dax-4k-holes.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> Subject: dax: fix deadlock with DAX 4k holes Currently in DAX if we have three read faults on the same hole address we can end up with the following: Thread 0 Thread 1 Thread 2 -------- -------- -------- dax_iomap_fault grab_mapping_entry lock_slot <locks empty DAX entry> dax_iomap_fault grab_mapping_entry get_unlocked_mapping_entry <sleeps on empty DAX entry> dax_iomap_fault grab_mapping_entry get_unlocked_mapping_entry <sleeps on empty DAX entry> dax_load_hole find_or_create_page ... page_cache_tree_insert dax_wake_mapping_entry_waiter <wakes one sleeper> __radix_tree_replace <swaps empty DAX entry with 4k zero page> <wakes> get_page lock_page ... put_locked_mapping_entry unlock_page put_page <sleeps forever on the DAX wait queue> The crux of the problem is that once we insert a 4k zero page, all locking from then on is done in terms of that 4k zero page and any additional threads sleeping on the empty DAX entry will never be woken. Fix this by waking all sleepers when we replace the DAX radix tree entry with a 4k zero page. This will allow all sleeping threads to successfully transition from locking based on the DAX empty entry to locking on the 4k zero page. With the test case reported by Xiong this happens very regularly in my test setup, with some runs resulting in 9+ threads in this deadlocked state. With this fix I've been able to run that same test dozens of times in a loop without issue. Fixes: ac401cc78242 ("dax: New fault locking") Link: http://lkml.kernel.org/r/1483479365-13607-1-git-send-email-ross.zwisler@xxxxxxxxxxxxxxx Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> Reported-by: Xiong Zhou <xzhou@xxxxxxxxxx> Reviewed-by: Jan Kara <jack@xxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> [4.7+] Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/filemap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff -puN mm/filemap.c~dax-fix-deadlock-with-dax-4k-holes mm/filemap.c --- a/mm/filemap.c~dax-fix-deadlock-with-dax-4k-holes +++ a/mm/filemap.c @@ -138,7 +138,7 @@ static int page_cache_tree_insert(struct dax_radix_locked_entry(0, RADIX_DAX_EMPTY)); /* Wakeup waiters for exceptional entry lock */ dax_wake_mapping_entry_waiter(mapping, page->index, p, - false); + true); } } __radix_tree_replace(&mapping->page_tree, node, slot, page, _ Patches currently in -mm which might be from ross.zwisler@xxxxxxxxxxxxxxx are dax-fix-deadlock-with-dax-4k-holes.patch mm-add-follow_pte_pmd.patch dax-wrprotect-pmd_t-in-dax_mapping_entry_mkclean.patch -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html