CVE-2026-23157
Unknown Unknown - Not Provided
Deadlock in Linux btrfs Due to Metadata Writeback Throttling

Publication date: 2026-02-14

Last updated on: 2026-03-25

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: btrfs: do not strictly require dirty metadata threshold for metadata writepages [BUG] There is an internal report that over 1000 processes are waiting at the io_schedule_timeout() of balance_dirty_pages(), causing a system hang and trigger a kernel coredump. The kernel is v6.4 kernel based, but the root problem still applies to any upstream kernel before v6.18. [CAUSE] From Jan Kara for his wisdom on the dirty page balance behavior first. This cgroup dirty limit was what was actually playing the role here because the cgroup had only a small amount of memory and so the dirty limit for it was something like 16MB. Dirty throttling is responsible for enforcing that nobody can dirty (significantly) more dirty memory than there's dirty limit. Thus when a task is dirtying pages it periodically enters into balance_dirty_pages() and we let it sleep there to slow down the dirtying. When the system is over dirty limit already (either globally or within a cgroup of the running task), we will not let the task exit from balance_dirty_pages() until the number of dirty pages drops below the limit. So in this particular case, as I already mentioned, there was a cgroup with relatively small amount of memory and as a result with dirty limit set at 16MB. A task from that cgroup has dirtied about 28MB worth of pages in btrfs btree inode and these were practically the only dirty pages in that cgroup. So that means the only way to reduce the dirty pages of that cgroup is to writeback the dirty pages of btrfs btree inode, and only after that those processes can exit balance_dirty_pages(). Now back to the btrfs part, btree_writepages() is responsible for writing back dirty btree inode pages. The problem here is, there is a btrfs internal threshold that if the btree inode's dirty bytes are below the 32M threshold, it will not do any writeback. This behavior is to batch as much metadata as possible so we won't write back those tree blocks and then later re-COW them again for another modification. This internal 32MiB is higher than the existing dirty page size (28MiB), meaning no writeback will happen, causing a deadlock between btrfs and cgroup: - Btrfs doesn't want to write back btree inode until more dirty pages - Cgroup/MM doesn't want more dirty pages for btrfs btree inode Thus any process touching that btree inode is put into sleep until the number of dirty pages is reduced. Thanks Jan Kara a lot for the analysis of the root cause. [ENHANCEMENT] Since kernel commit b55102826d7d ("btrfs: set AS_KERNEL_FILE on the btree_inode"), btrfs btree inode pages will only be charged to the root cgroup which should have a much larger limit than btrfs' 32MiB threshold. So it should not affect newer kernels. But for all current LTS kernels, they are all affected by this problem, and backporting the whole AS_KERNEL_FILE may not be a good idea. Even for newer kernels I still think it's a good idea to get rid of the internal threshold at btree_writepages(), since for most cases cgroup/MM has a better view of full system memory usage than btrfs' fixed threshold. For internal callers using btrfs_btree_balance_dirty() since that function is already doing internal threshold check, we don't need to bother them. But for external callers of btree_writepages(), just respect their requests and write back whatever they want, ignoring the internal btrfs threshold to avoid such deadlock on btree inode dirty page balancing.
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Meta Information
Published
2026-02-14
Last Modified
2026-03-25
Generated
2026-05-07
AI Q&A
2026-02-14
EPSS Evaluated
2026-05-05
NVD
CVE-2026-23157
EUVD
EUVD-2026-6113
Affected Vendors & Products
Showing 8 associated CPEs
Vendor Product Version / Range
linux linux_kernel 6.19
linux linux_kernel 6.19
linux linux_kernel 6.19
linux linux_kernel 6.19
linux linux_kernel 6.19
linux linux_kernel 6.19
linux linux_kernel 6.19
linux linux_kernel From 2.6.29 (inc) to 6.18.9 (exc)
Helpful Resources
Exploitability
CWE
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KEV
KEV Icon
CWE ID Description
CWE-667 The product does not properly acquire or release a lock on a resource, leading to unexpected resource state changes and behaviors.
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AI Powered Q&A
Can you explain this vulnerability to me?

This vulnerability exists in the Linux kernel's btrfs filesystem, specifically related to how dirty metadata pages are written back. The issue arises because btrfs has an internal threshold of 32MiB for dirty btree inode pages before it triggers a writeback. However, if the dirty pages are below this threshold but above the cgroup's dirty memory limit (e.g., 16MB), processes get stuck waiting because btrfs refuses to write back the dirty pages and the cgroup refuses to allow more dirty pages. This causes a deadlock where over 1000 processes can hang at io_schedule_timeout() in balance_dirty_pages(), potentially leading to a system hang and kernel coredump.

The root cause is the mismatch between the btrfs internal threshold and the cgroup dirty memory limit, causing processes to sleep indefinitely waiting for dirty pages to be written back, which never happens due to the threshold logic.

This problem affects Linux kernels before version 6.18, and while newer kernels have improvements to mitigate this by charging btree inode pages to the root cgroup with a larger limit, older LTS kernels remain vulnerable.


How can this vulnerability impact me? :

This vulnerability can cause a system hang due to a deadlock situation where many processes (over 1000) are stuck waiting for dirty metadata pages to be written back. This can lead to a kernel coredump, effectively causing a denial of service (DoS) on the affected system.

Systems running affected Linux kernel versions (before 6.18) with btrfs filesystems and cgroups configured with small memory limits are particularly at risk. The deadlock can severely degrade system performance or cause complete unresponsiveness.


How does this vulnerability affect compliance with common standards and regulations (like GDPR, HIPAA)?:

I don't know


How can this vulnerability be detected on my network or system? Can you suggest some commands?

I don't know


What immediate steps should I take to mitigate this vulnerability?

I don't know


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