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SubjectRe: [PATCH] btrfs: fix allocation of bitmap pages.
On Sat, Aug 17, 2019 at 07:44:39AM +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.

This assumption is not documented nor any pitfalls mentioned in
include/asm-generic/page.h that provides the generic implementation. I
as an API user cannot check each arch implementation for additional
constraints or I would expect that it deals with the boundary cases the
same way as arch-specific memcpy implementations.

Another thing that is lost is the slub debugging support for all
architectures, because get_zeroed_pages lacking the red zones and sanity
checks.

I find working with raw pages in this code a bit inconsistent with the
rest of btrfs code, but that's rather minor compared to the above.

Summing it up, I think that the proper fix should go to copy_page
implementation on architectures that require it or make it clear what
are the copy_page constraints.

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