On 2018/10/26 下午7:41, Nikolay Borisov wrote:
> Running btrfs/124 in a loop hung up on me sporadically with the
> following call trace:
> btrfs D 0 5760 5324 0x00000000
> Call Trace:
> ? __schedule+0x243/0x800
> schedule+0x33/0x90
> btrfs_start_ordered_extent+0x10c/0x1b0 [btrfs]
> ? wait_woken+0xa0/0xa0
> btrfs_wait_ordered_range+0xbb/0x100 [btrfs]
> btrfs_relocate_block_group+0x1ff/0x230 [btrfs]
> btrfs_relocate_chunk+0x49/0x100 [btrfs]
> btrfs_balance+0xbeb/0x1740 [btrfs]
> btrfs_ioctl_balance+0x2ee/0x380 [btrfs]
> btrfs_ioctl+0x1691/0x3110 [btrfs]
> ? lockdep_hardirqs_on+0xed/0x180
> ? __handle_mm_fault+0x8e7/0xfb0
> ? _raw_spin_unlock+0x24/0x30
> ? __handle_mm_fault+0x8e7/0xfb0
> ? do_vfs_ioctl+0xa5/0x6e0
> ? btrfs_ioctl_get_supported_features+0x30/0x30 [btrfs]
> do_vfs_ioctl+0xa5/0x6e0
> ? entry_SYSCALL_64_after_hwframe+0x3e/0xbe
> ksys_ioctl+0x3a/0x70
> __x64_sys_ioctl+0x16/0x20
> do_syscall_64+0x60/0x1b0
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> Turns out during page writeback it's possible that the code in
> writepage_delalloc can instantiate a delalloc range which doesn't
> belong to the page currently being written back.
Just a nitpick, would you please split long paragraphs with newlines?
It makes reviewers' eyes less painful.
> This happens since
> find_lock_delalloc_range returns up to BTRFS_MAX_EXTENT_SIZE delalloc
> range when asked and doens't really consider the range of the passed
> page.
> When such a foregin range is found the code proceeds to
> run_delalloc_range and calls the appropriate function to fill the
> delalloc and create ordered extents. If, however, a failure occurs
> while this operation is in effect then the clean up code in
> btrfs_cleanup_ordered_extents will be called. This function has the
> wrong assumption that caller of run_delalloc_range always properly
> cleans the first page of the range hence when it calls
> __endio_write_update_ordered it explicitly ommits the first page of
> the delalloc range.
Yes, that's the old assumption, at least well explained by some ascii
art. (even it's wrong)
> This is wrong because this function could be
> cleaning a delalloc range that doesn't belong to the current page. This
> in turn means that the page cleanup code in __extent_writepage will
> not really free the initial page for the range, leaving a hanging
> ordered extent with bytes_left set to 4k. This bug has been present
> ever since the original introduction of the cleanup code in 524272607e88.
The cause sounds valid, however would you please explain more about how
such cleaning on unrelated delalloc range happens?
Even the fix looks solid, it's still better to explain the cause a
little more, as the more we understand the cause, the better solution we
may get.
>
> Fix this by correctly checking whether the current page belongs to the
> range being instantiated and if so correctly adjust the range parameters
> passed for cleaning up. If it doesn't, then just clean the whole OE
> range directly.
And the solution also looks good to me, and much more robust, without
any (possibly wrong) assumption.
Thanks,
Qu
>
> Signed-off-by: Nikolay Borisov <nbori...@suse.com>
> Fixes: 524272607e88 ("btrfs: Handle delalloc error correctly to avoid ordered
> extent hang")
> ---
>
> V2:
> * Fix compilation failure due to missing parentheses
> * Fixed the "Fixes" tag.
>
> fs/btrfs/inode.c | 29 ++++++++++++++++++++---------
> 1 file changed, 20 insertions(+), 9 deletions(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index e1f00d8c24ce..5906564ae2e9 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -109,17 +109,17 @@ static void __endio_write_update_ordered(struct inode
> *inode,
> * extent_clear_unlock_delalloc() to clear both the bits EXTENT_DO_ACCOUNTING
> * and EXTENT_DELALLOC simultaneously, because that causes the reserved
> metadata
> * to be released, which we want to happen only when finishing the ordered
> - * extent (btrfs_finish_ordered_io()). Also note that the caller of the
> - * fill_delalloc() callback already does proper cleanup for the first page of
> - * the range, that is, it invokes the callback writepage_end_io_hook() for
> the
> - * range of the first page.
> + * extent (btrfs_finish_ordered_io()).
> */
> static inline void btrfs_cleanup_ordered_extents(struct inode *inode,
> - const u64 offset,
> - const u64 bytes)
> + struct page *locked_page,
> + u64 offset, u64 bytes)
> {
> unsigned long index = offset >> PAGE_SHIFT;
> unsigned long end_index = (offset + bytes - 1) >> PAGE_SHIFT;
> + u64 page_start = page_offset(locked_page);
> + u64 page_end = page_start + PAGE_SIZE - 1;
> +
> struct page *page;
>
> while (index <= end_index) {
> @@ -130,8 +130,18 @@ static inline void btrfs_cleanup_ordered_extents(struct
> inode *inode,
> ClearPagePrivate2(page);
> put_page(page);
> }
> - return __endio_write_update_ordered(inode, offset + PAGE_SIZE,
> - bytes - PAGE_SIZE, false);
> +
> + /*
> + * In case this page belongs to the delalloc range being instantiated
> + * then skip it, since the first page of a range is going to be
> + * properly cleaned up by the caller of run_delalloc_range
> + */
> + if (page_start >= offset && page_end <= (offset + bytes - 1)) {
> + offset += PAGE_SIZE;
> + bytes -= PAGE_SIZE;
> + }
> +
> + return __endio_write_update_ordered(inode, offset, bytes, false);
> }
>
> static int btrfs_dirty_inode(struct inode *inode);
> @@ -1606,7 +1616,8 @@ static int run_delalloc_range(void *private_data,
> struct page *locked_page,
> write_flags);
> }
> if (ret)
> - btrfs_cleanup_ordered_extents(inode, start, end - start + 1);
> + btrfs_cleanup_ordered_extents(inode, locked_page, start,
> + end - start + 1);
> return ret;
> }
>
>
