On Sat, Jul 16, 2016 at 4:45 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
> On Fri, Jul 15, 2016 at 10:00 PM, Ross Zwisler
> <ross.zwisler@xxxxxxxxxxxxxxx> wrote:
>> On Fri, Jul 15, 2016 at 11:52:58AM +0300, Andrey Ryabinin wrote:
>>> On 07/15/2016 01:25 AM, Ross Zwisler wrote:
>>> > On Thu, Jul 14, 2016 at 02:19:56PM +0300, Andrey Ryabinin wrote:
>>> >> radix_tree_iter_retry() resets slot to NULL, but it doesn't reset tags.
>>> >> Then NULL slot and non-zero iter.tags passed to radix_tree_next_slot()
>>> >> leading to crash:
>>> >>
>>> >> RIP: [< inline >] radix_tree_next_slot include/linux/radix-tree.h:473
>>> >> [<ffffffff816951a4>] find_get_pages_tag+0x334/0x930 mm/filemap.c:1452
>>> >> ....
>>> >> Call Trace:
>>> >> [<ffffffff816cd91a>] pagevec_lookup_tag+0x3a/0x80 mm/swap.c:960
>>> >> [<ffffffff81ab4231>] mpage_prepare_extent_to_map+0x321/0xa90 fs/ext4/inode.c:2516
>>> >> [<ffffffff81ac883e>] ext4_writepages+0x10be/0x2b20 fs/ext4/inode.c:2736
>>> >> [<ffffffff816c99c7>] do_writepages+0x97/0x100 mm/page-writeback.c:2364
>>> >> [<ffffffff8169bee8>] __filemap_fdatawrite_range+0x248/0x2e0 mm/filemap.c:300
>>> >> [<ffffffff8169c371>] filemap_write_and_wait_range+0x121/0x1b0 mm/filemap.c:490
>>> >> [<ffffffff81aa584d>] ext4_sync_file+0x34d/0xdb0 fs/ext4/fsync.c:115
>>> >> [<ffffffff818b667a>] vfs_fsync_range+0x10a/0x250 fs/sync.c:195
>>> >> [< inline >] vfs_fsync fs/sync.c:209
>>> >> [<ffffffff818b6832>] do_fsync+0x42/0x70 fs/sync.c:219
>>> >> [< inline >] SYSC_fdatasync fs/sync.c:232
>>> >> [<ffffffff818b6f89>] SyS_fdatasync+0x19/0x20 fs/sync.c:230
>>> >> [<ffffffff86a94e00>] entry_SYSCALL_64_fastpath+0x23/0xc1 arch/x86/entry/entry_64.S:207
>>> >>
>>> >> We must reset iterator's tags to bail out from radix_tree_next_slot() and
>>> >> go to the slow-path in radix_tree_next_chunk().
>>> >
>>> > This analysis doesn't make sense to me. In find_get_pages_tag(), when we call
>>> > radix_tree_iter_retry(), this sets the local 'slot' variable to NULL, then
>>> > does a 'continue'. This will hop to the next iteration of the
>>> > radix_tree_for_each_tagged() loop, which will very check the exit condition of
>>> > the for() loop:
>>> >
>>> > #define radix_tree_for_each_tagged(slot, root, iter, start, tag) \
>>> > for (slot = radix_tree_iter_init(iter, start) ; \
>>> > slot || (slot = radix_tree_next_chunk(root, iter, \
>>> > RADIX_TREE_ITER_TAGGED | tag)) ; \
>>> > slot = radix_tree_next_slot(slot, iter, \
>>> > RADIX_TREE_ITER_TAGGED))
>>> >
>>> > So, we'll run the
>>> > slot || (slot = radix_tree_next_chunk(root, iter, \
>>> > RADIX_TREE_ITER_TAGGED | tag)) ; \
>>> >
>>> > bit first.
>>>
>>> This is not the way how the for() loop works. slot = radix_tree_next_slot() executed first
>>> and only after that goes the condition statement.
>>
>> Right...*sigh*... Thanks for the sanity check. :)
>>
>>> > 'slot' is NULL, so we'll set it via radix_tree_next_chunk(). At
>>> > this point radix_tree_next_slot() hasn't been called.
>>> >
>>> > radix_tree_next_chunk() will set up the iter->index, iter->next_index and
>>> > iter->tags before it returns. The next iteration of the loop in
>>> > find_get_pages_tag() will use the non-NULL slot provided by
>>> > radix_tree_next_chunk(), and only after that iteration will we call
>>> > radix_tree_next_slot() again. By then iter->tags should be up to date.
>>> >
>>> > Do you have a test setup that reliably fails without this code but passes when
>>> > you zero out iter->tags?
>>> >
>>>
>>>
>>> Yup, I run Dmitry's reproducer in a parallel loop:
>>> $ while true; do ./a.out & done
>>>
>>> Usually it takes just couple minutes maximum.
>>
>> Cool - I was able to get this to work on my system as well by upping the
>> thread count.
>>
>> In looking at this more, I agree that your patch fixes this particular bug,
>> but I think that ultimately we might want something more general.
>>
>> IIUC, the real issue is that we shouldn't be running through
>> radix_tree_next_slot() with a NULL 'slot' parameter. In the end I think it's
>> fine to zero out iter->tags in radix_tree_iter_retry(), but really we want to
>> guarantee that we just bail out of radix_tree_next_slot() if we have a NULL
>> 'slot'.
>>
>> I've run this patch in my test setup, and it fixes the reproducer provided by
>> Dmitry. I've also run xfstests against it with out any failures.
>>
>> --- 8< ---
>> From 533beefac12f61f467aeb72e2d2c46685247b9bc Mon Sep 17 00:00:00 2001
>> From: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> Date: Fri, 15 Jul 2016 12:46:38 -0600
>> Subject: [PATCH] radix-tree: 'slot' can be NULL in radix_tree_next_slot()
>>
>> There are four cases I can see where we could end up with a NULL 'slot' in
>> radix_tree_next_slot() (there might be more):
>>
>> 1) radix_tree_iter_retry() via a non-tagged iteration like
>> radix_tree_for_each_slot(). In this case we currently aren't seeing a bug
>> because radix_tree_iter_retry() sets
>>
>> iter->next_index = iter->index;
>>
>> which means that in in the else case in radix_tree_next_slot(), 'count' is
>> zero, so we skip over the while() loop and effectively just return NULL
>> without ever dereferencing 'slot'.
>>
>> 2) radix_tree_iter_retry() via tagged iteration like
>> radix_tree_for_each_tagged(). With the current code this case is
>> unhandled and we have seen it result in a kernel crash when we dereference
>> 'slot'.
>>
>> 3) radix_tree_iter_next() via via a non-tagged iteration like
>> radix_tree_for_each_slot(). This currently happens in shmem_tag_pins()
>> and shmem_partial_swap_usage().
>>
>> I think that this case is currently unhandled. Unlike with
>> radix_tree_iter_retry() case (#1 above) we can't rely on 'count' in the else
>> case of radix_tree_next_slot() to be zero, so I think it's possible we'll end
>> up executing code in the while() loop in radix_tree_next_slot() that assumes
>> 'slot' is valid.
>>
>> I haven't actually seen this crash on a test setup, but I don't think the
>> current code is safe.
>
> This is becase distance between ->index and ->next_index now could be
> more that one?
>
> We could fix that by adding "iter->index = iter->next_index - 1;" into
> radix_tree_iter_next()
> right after updating next_index and tweak multi-order itreration logic
> if it depends on that.
>
> I'd like to keep radix_tree_next_slot() as small as possible because
> this is supposed to be a fast-path.
Support of multi-order entries in iterator is ridiculously over-engineered.
If radix_tree_next_chunk() finds multi-order entry it must return chunk
with size 1, radix_tree_next_slot() should know nothing about that.
I'll try to fix that.
>
>>
>> 4) radix_tree_iter_next() via tagged iteration like
>> radix_tree_for_each_tagged(). This happens in shmem_wait_for_pins().
>>
>> radix_tree_iter_next() zeros out iter->tags, so we end up exiting
>> radix_tree_next_slot() here:
>>
>> if (flags & RADIX_TREE_ITER_TAGGED) {
>> void *canon = slot;
>>
>> iter->tags >>= 1;
>> if (unlikely(!iter->tags))
>> return NULL;
>>
>> Really we want to guarantee that we just bail out of
>> radix_tree_next_slot() if we have a NULL 'slot'. This is a more explicit
>> way of handling all the 4 above cases.
>>
>> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> ---
>> include/linux/radix-tree.h | 3 +++
>> 1 file changed, 3 insertions(+)
>>
>> diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
>> index cb4b7e8..840308d 100644
>> --- a/include/linux/radix-tree.h
>> +++ b/include/linux/radix-tree.h
>> @@ -463,6 +463,9 @@ static inline struct radix_tree_node *entry_to_node(void *ptr)
>> static __always_inline void **
>> radix_tree_next_slot(void **slot, struct radix_tree_iter *iter, unsigned flags)
>> {
>> + if (unlikely(!slot))
>> + return NULL;
>> +
>> if (flags & RADIX_TREE_ITER_TAGGED) {
>> void *canon = slot;
>>
>> --
>> 2.9.0
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