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If allocation is racy with swapoff, we may call free_cluster for cluster
already in free list and trigger BUG_ON() as following:
Allocation Swapoff
cluster_alloc_swap_entry
...
/* may get a free cluster with offset */
offset = xxx;
if (offset)
ci = lock_cluster(si, offset);
...
del_from_avail_list(p, true);
si->flags &= ~SWP_WRITEOK;
alloc_swap_scan_cluster(si, ci, ...)
...
/* failed to alloc entry from free entry */
if (!cluster_alloc_range(...))
break;
...
/* add back a free cluster */
relocate_cluster(si, ci);
if (!ci->count)
free_cluster(si, ci);
VM_BUG_ON(ci->flags == CLUSTER_FLAG_FREE);
To prevent the BUG_ON(), call free_cluster() for free cluster to move the
cluster to tail of list.
Check cluster is not free before calling free_cluster() in
relocate_cluster() to avoid BUG_ON().
Link: https://lkml.kernel.org/r/20250222160850.505274-4-shikemeng@huaweicloud.com
Fixes: 3b644773ee ("mm, swap: reduce contention on device lock")
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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.