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2706659d64[BUG] There is a bug report that during emergency sync, btrfs only write back all the dirty data and metadadta, but no full transaction commit, resulting the super block still pointing to the old trees, thus the end user can only see the old data, not the newer one. [CAUSE] Initially this looks like a btrfs specific bug, since ext4 doesn't get affected by this one. But the root problem here is, a combination of btrfs features and the no wait nature of emergency sync. Firstly do_sync_work() will call sync_inodes_one_sb() for every fs, to writeback all the dirty pages for the fs. Btrfs will properly writeback all dirty pages, including both data and the updated metadata. So far so good. Then sync_fs_one_sb() called with @nowait, in the case of btrfs it means no full transaction commit, thus no super block update. At this stage, btrfs is only one super block update away to be fully committed. I believe it's the more or less the same for other fses too. The problem is the next step, sync_bdevs(). Normally other fses have their super block already updated in the page cache of the block device, but btrfs only updates the super block during full transaction commit. So sync_bdevs() may work for other fses, but not for btrfs, btrfs is still using its older super block, all pointing back to the old metadata and data. Thus if after emergency sync, power loss happened, the end user will only see the old data, not the newer one, despite that everything but the super block is already written back. [FIX] Since the emergency sync is already executing in a workqueue, I didn't see much need to only do a nowait sync. Especially after the fact that sync_inodes_one_sb() always wait for the writeback to finish. Instead for the second iteration of sync_fs_one_sb(), pass wait == 1 into it, so fses like btrfs can properly commit its super blocks. Reported-by: Askar Safin <safinaskar@gmail.com> Link: https://lore.kernel.org/linux-btrfs/20251101150429.321537-1-safinaskar@gmail.com/ Signed-off-by: Qu Wenruo <wqu@suse.com> Link: https://patch.msgid.link/7b7fd40c5fe440b633b6c0c741d96ce93eb5a89a.1762142636.git.wqu@suse.com Signed-off-by: Christian Brauner <brauner@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.