| Message ID | 20190614003350.1178444-7-tj@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/8] blkcg, writeback: Add wbc->no_wbc_acct | expand |
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
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c index 13fca7bfc1f2..9f223d7d78c0 100644 --- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c @@ -1838,7 +1838,7 @@ static int __process_pages_contig(struct address_space *mapping, if (page_ops & PAGE_SET_PRIVATE2) SetPagePrivate2(pages[i]); - if (pages[i] == locked_page) { + if (locked_page && pages[i] == locked_page) { put_page(pages[i]); pages_locked++; continue; diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index 91b161fb1521..df5527cc07b9 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -666,10 +666,12 @@ static noinline void compress_file_range(struct async_chunk *async_chunk, * to our extent and set things up for the async work queue to run * cow_file_range to do the normal delalloc dance. */ - if (page_offset(async_chunk->locked_page) >= start && - page_offset(async_chunk->locked_page) <= end) + if (async_chunk->locked_page && + (page_offset(async_chunk->locked_page) >= start && + page_offset(async_chunk->locked_page)) <= end) { __set_page_dirty_nobuffers(async_chunk->locked_page); /* unlocked later on in the async handlers */ + } if (redirty) extent_range_redirty_for_io(inode, start, end); @@ -759,7 +761,7 @@ static noinline void submit_compressed_extents(struct async_chunk *async_chunk) async_extent->start + async_extent->ram_size - 1, WB_SYNC_ALL); - else if (ret) + else if (ret && async_chunk->locked_page) unlock_page(async_chunk->locked_page); kfree(async_extent); cond_resched(); @@ -1236,10 +1238,25 @@ static int cow_file_range_async(struct inode *inode, struct page *locked_page, async_chunk[i].inode = inode; async_chunk[i].start = start; async_chunk[i].end = cur_end; - async_chunk[i].locked_page = locked_page; async_chunk[i].write_flags = write_flags; INIT_LIST_HEAD(&async_chunk[i].extents); + /* + * The locked_page comes all the way from writepage and its + * the original page we were actually given. As we spread + * this large delalloc region across multiple async_cow + * structs, only the first struct needs a pointer to locked_page + * + * This way we don't need racey decisions about who is supposed + * to unlock it. + */ + if (locked_page) { + async_chunk[i].locked_page = locked_page; + locked_page = NULL; + } else { + async_chunk[i].locked_page = NULL; + } + btrfs_init_work(&async_chunk[i].work, btrfs_delalloc_helper, async_cow_start, async_cow_submit,