Re: [PATCH 6/8] Btrfs: only associate the locked page with one async_cow struct

Josef Bacik
2019-06-14T13:49:05Z

On Thu, Jun 13, 2019 at 05:33:48PM -0700, Tejun Heo wrote:
> From: Chris Mason <clm@fb.com>
>
> The btrfs writepages function collects a large range of pages flagged
> for delayed allocation, and then sends them down through the COW code
> for processing. When compression is on, we allocate one async_cow
> structure for every 512K, and then run those pages through the
> compression code for IO submission.
>
> writepages starts all of this off with a single page, locked by
> the original call to extent_write_cache_pages(), and it's important to
> keep track of this page because it has already been through
> clear_page_dirty_for_io().
>
> The btrfs async_cow struct has a pointer to the locked_page, and when
> we're redirtying the page because compression had to fallback to
> uncompressed IO, we use page->index to decide if a given async_cow
> struct really owns that page.
>
> But, this is racey. If a given delalloc range is broken up into two
> async_cows (cow_A and cow_B), we can end up with something like this:
>
> compress_file_range(cowA)
> submit_compress_extents(cowA)
> submit compressed bios(cowA)
> put_page(locked_page)
>
> compress_file_range(cowB)
> ...
>
> The end result is that cowA is completed and cleaned up before cowB even
> starts processing. This means we can free locked_page() and reuse it
> elsewhere. If we get really lucky, it'll have the same page->index in
> its new home as it did before.
>
> While we're processing cowB, we might decide we need to fall back to
> uncompressed IO, and so compress_file_range() will call
> __set_page_dirty_nobufers() on cowB->locked_page.
>
> Without cgroups in use, this creates as a phantom dirty page, which
> isn't great but isn't the end of the world. With cgroups in use, we
> might crash in the accounting code because page->mapping->i_wb isn't
> set.
>
> [ 8308.523110] BUG: unable to handle kernel NULL pointer dereference at 00000000000000d0
> [ 8308.531084] IP: percpu_counter_add_batch+0x11/0x70
> [ 8308.538371] PGD 66534e067 P4D 66534e067 PUD 66534f067 PMD 0
> [ 8308.541750] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
> [ 8308.551948] CPU: 16 PID: 2172 Comm: rm Not tainted
> [ 8308.566883] RIP: 0010:percpu_counter_add_batch+0x11/0x70
> [ 8308.567891] RSP: 0018:ffffc9000a97bbe0 EFLAGS: 00010286
> [ 8308.568986] RAX: 0000000000000005 RBX: 0000000000000090 RCX: 0000000000026115
> [ 8308.570734] RDX: 0000000000000030 RSI: ffffffffffffffff RDI: 0000000000000090
> [ 8308.572543] RBP: 0000000000000000 R08: fffffffffffffff5 R09: 0000000000000000
> [ 8308.573856] R10: 00000000000260c0 R11: ffff881037fc26c0 R12: ffffffffffffffff
> [ 8308.580099] R13: ffff880fe4111548 R14: ffffc9000a97bc90 R15: 0000000000000001
> [ 8308.582520] FS: 00007f5503ced480(0000) GS:ffff880ff7200000(0000) knlGS:0000000000000000
> [ 8308.585440] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 8308.587951] CR2: 00000000000000d0 CR3: 00000001e0459005 CR4: 0000000000360ee0
> [ 8308.590707] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 8308.592865] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 8308.594469] Call Trace:
> [ 8308.595149] account_page_cleaned+0x15b/0x1f0
> [ 8308.596340] __cancel_dirty_page+0x146/0x200
> [ 8308.599395] truncate_cleanup_page+0x92/0xb0
> [ 8308.600480] truncate_inode_pages_range+0x202/0x7d0
> [ 8308.617392] btrfs_evict_inode+0x92/0x5a0
> [ 8308.619108] evict+0xc1/0x190
> [ 8308.620023] do_unlinkat+0x176/0x280
> [ 8308.621202] do_syscall_64+0x63/0x1a0
> [ 8308.623451] entry_SYSCALL_64_after_hwframe+0x42/0xb7
>
> The fix here is to make asyc_cow->locked_page NULL everywhere but the
> one async_cow struct that's allowed to do things to the locked page.
>
> Signed-off-by: Chris Mason <clm@fb.com>
> Fixes: 771ed689d2cd ("Btrfs: Optimize compressed writeback and reads")
> ---

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef