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Subject[PATCH 5.4 180/191] Btrfs: only associate the locked page with one async_chunk struct
Date
From: Chris Mason <clm@fb.com>

[ Upstream commit 1d53c9e6723022b12e4a5ed4b141f67c834b7f6f ]

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_chunk
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_chunk 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_chunk
struct really owns that page.

But, this is racey. If a given delalloc range is broken up into two
async_chunks (chunkA and chunkB), we can end up with something like
this:

compress_file_range(chunkA)
submit_compress_extents(chunkA)
submit compressed bios(chunkA)
put_page(locked_page)

compress_file_range(chunkB)
...

Or:

async_cow_submit
submit_compressed_extents <--- falls back to buffered writeout
cow_file_range
extent_clear_unlock_delalloc
__process_pages_contig
put_page(locked_pages)

async_cow_submit

The end result is that chunkA is completed and cleaned up before chunkB
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 chunkB, we might decide we need to fall back to
uncompressed IO, and so compress_file_range() will call
__set_page_dirty_nobufers() on chunkB->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. What can happen, it can go
through the fixup worker and the whole COW machinery again:

in submit_compressed_extents():
while (async extents) {
...
cow_file_range
if (!page_started ...)
extent_write_locked_range
else if (...)
unlock_page
continue;

This hasn't been observed in practice but is still possible.

With cgroups in use, we might crash in the accounting code because
page->mapping->i_wb isn't set.

BUG: unable to handle kernel NULL pointer dereference at 00000000000000d0
IP: percpu_counter_add_batch+0x11/0x70
PGD 66534e067 P4D 66534e067 PUD 66534f067 PMD 0
Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
CPU: 16 PID: 2172 Comm: rm Not tainted
RIP: 0010:percpu_counter_add_batch+0x11/0x70
RSP: 0018:ffffc9000a97bbe0 EFLAGS: 00010286
RAX: 0000000000000005 RBX: 0000000000000090 RCX: 0000000000026115
RDX: 0000000000000030 RSI: ffffffffffffffff RDI: 0000000000000090
RBP: 0000000000000000 R08: fffffffffffffff5 R09: 0000000000000000
R10: 00000000000260c0 R11: ffff881037fc26c0 R12: ffffffffffffffff
R13: ffff880fe4111548 R14: ffffc9000a97bc90 R15: 0000000000000001
FS: 00007f5503ced480(0000) GS:ffff880ff7200000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000000d0 CR3: 00000001e0459005 CR4: 0000000000360ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
account_page_cleaned+0x15b/0x1f0
__cancel_dirty_page+0x146/0x200
truncate_cleanup_page+0x92/0xb0
truncate_inode_pages_range+0x202/0x7d0
btrfs_evict_inode+0x92/0x5a0
evict+0xc1/0x190
do_unlinkat+0x176/0x280
do_syscall_64+0x63/0x1a0
entry_SYSCALL_64_after_hwframe+0x42/0xb7

The fix here is to make asyc_chunk->locked_page NULL everywhere but the
one async_chunk struct that's allowed to do things to the locked page.

Link: https://lore.kernel.org/linux-btrfs/c2419d01-5c84-3fb4-189e-4db519d08796@suse.com/
Fixes: 771ed689d2cd ("Btrfs: Optimize compressed writeback and reads")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Chris Mason <clm@fb.com>
[ update changelog from mail thread discussion ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/extent_io.c | 2 +-
fs/btrfs/inode.c | 25 +++++++++++++++++++++----
2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index be9dc78aa727..33c6b191ca59 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1899,7 +1899,7 @@ 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 dc14fc2e4206..0b2758961b1c 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -712,10 +712,12 @@ cleanup_and_bail_uncompressed:
* 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);
@@ -795,7 +797,7 @@ retry:
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();
@@ -1264,10 +1266,25 @@ 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_chunk
+ * 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, async_cow_start,
async_cow_submit, async_cow_free);

--
2.20.1


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