Re: [PATCH v2] btrfs: fix allocation of bitmap pages.

[Nikolay Borisov]

On 26.08.19 Ð. 18:37 Ñ., David Sterba wrote:
> On Wed, Aug 21, 2019 at 03:05:55PM +0000, Christophe Leroy wrote:
>> Various notifications of type "BUG kmalloc-4096 () : Redzone
>> overwritten" have been observed recently in various parts of
>> the kernel. After some time, it has been made a relation with
>> the use of BTRFS filesystem.
>>
>> [   22.809700] BUG kmalloc-4096 (Tainted: G        W        ): Redzone overwritten
>> [   22.809971] -----------------------------------------------------------------------------
>>
>> [   22.810286] INFO: 0xbe1a5921-0xfbfc06cd. First byte 0x0 instead of 0xcc
>> [   22.810866] INFO: Allocated in __load_free_space_cache+0x588/0x780 [btrfs] age=22 cpu=0 pid=224
>> [   22.811193] 	__slab_alloc.constprop.26+0x44/0x70
>> [   22.811345] 	kmem_cache_alloc_trace+0xf0/0x2ec
>> [   22.811588] 	__load_free_space_cache+0x588/0x780 [btrfs]
>> [   22.811848] 	load_free_space_cache+0xf4/0x1b0 [btrfs]
>> [   22.812090] 	cache_block_group+0x1d0/0x3d0 [btrfs]
>> [   22.812321] 	find_free_extent+0x680/0x12a4 [btrfs]
>> [   22.812549] 	btrfs_reserve_extent+0xec/0x220 [btrfs]
>> [   22.812785] 	btrfs_alloc_tree_block+0x178/0x5f4 [btrfs]
>> [   22.813032] 	__btrfs_cow_block+0x150/0x5d4 [btrfs]
>> [   22.813262] 	btrfs_cow_block+0x194/0x298 [btrfs]
>> [   22.813484] 	commit_cowonly_roots+0x44/0x294 [btrfs]
>> [   22.813718] 	btrfs_commit_transaction+0x63c/0xc0c [btrfs]
>> [   22.813973] 	close_ctree+0xf8/0x2a4 [btrfs]
>> [   22.814107] 	generic_shutdown_super+0x80/0x110
>> [   22.814250] 	kill_anon_super+0x18/0x30
>> [   22.814437] 	btrfs_kill_super+0x18/0x90 [btrfs]
>> [   22.814590] INFO: Freed in proc_cgroup_show+0xc0/0x248 age=41 cpu=0 pid=83
>> [   22.814841] 	proc_cgroup_show+0xc0/0x248
>> [   22.814967] 	proc_single_show+0x54/0x98
>> [   22.815086] 	seq_read+0x278/0x45c
>> [   22.815190] 	__vfs_read+0x28/0x17c
>> [   22.815289] 	vfs_read+0xa8/0x14c
>> [   22.815381] 	ksys_read+0x50/0x94
>> [   22.815475] 	ret_from_syscall+0x0/0x38
>>
>> Commit 69d2480456d1 ("btrfs: use copy_page for copying pages instead
>> of memcpy") changed the way bitmap blocks are copied. But allthough
>> bitmaps have the size of a page, they were allocated with kzalloc().
>>
>> Most of the time, kzalloc() allocates aligned blocks of memory, so
>> copy_page() can be used. But when some debug options like SLAB_DEBUG
>> are activated, kzalloc() may return unaligned pointer.
>>
>> On powerpc, memcpy(), copy_page() and other copying functions use
>> 'dcbz' instruction which provides an entire zeroed cacheline to avoid
>> memory read when the intention is to overwrite a full line. Functions
>> like memcpy() are writen to care about partial cachelines at the start
>> and end of the destination, but copy_page() assumes it gets pages. As
>> pages are naturally cache aligned, copy_page() doesn't care about
>> partial lines. This means that when copy_page() is called with a
>> misaligned pointer, a few leading bytes are zeroed.
>>
>> To fix it, allocate bitmaps through kmem_cache instead of using kzalloc()
>> The cache pool is created with PAGE_SIZE alignment constraint.
>>
>> Reported-by: Erhard F. <erhard_f@xxxxxxxxxxx>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=204371
>> Fixes: 69d2480456d1 ("btrfs: use copy_page for copying pages instead of memcpy")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
>> ---
>> v2: Using kmem_cache instead of get_zeroed_page() in order to benefit from SLAB debugging features like redzone.
> 
> I'll take this version, thanks. Though I'm not happy about the allocator
> behaviour. The kmem cache based fix can be backported independently to
> 4.19 regardless of the SL*B fixes.
> 
>> +extern struct kmem_cache *btrfs_bitmap_cachep;
> 
> I've renamed the cache to btrfs_free_space_bitmap_cachep
> 
> Reviewed-by: David Sterba <dsterba@xxxxxxxx>

Isn't this obsoleted by

'[PATCH v2 0/2] guarantee natural alignment for kmalloc()' ?

>