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commit2788cf0c40("memcg: reparent list_lrus and free kmemcg_id on css offline") removed sanity checks for the nr_items counter's value because it implemented list_lru re-parenting in a way that will redirect children's list_lru to the parent before re-parenting the items in list_lru. This will make item counter uncharging happen in the parent while the item is still being held by the child. As a result, the parent's counter value may become negative. This is acceptable because re-parenting will sum up the children's counter values, and the parent's counter will be fixed. Later commitfb56fdf8b9("mm/list_lru: split the lock to per-cgroup scope") reworked the re-parenting process, and removed the redirect. So it added the sanity check back, assuming that as long as items are still in the children's list_lru, parent's counter will not be uncharged. But that assumption is incorrect. The xas_store in memcg_reparent_list_lrus will set children's list_lru to NULL before re-parenting the items, it redirects list_lru helpers to use parent's list_lru just like before. But still, it's not a problem as re-parenting will fix the counter. Therefore, remove this sanity check, but add a new check to ensure that the counter won't go negative in a different way: the child's list_lru being re-parented should never have a negative counter, since re-parenting should occur in order and fixes counters. Link: https://lkml.kernel.org/r/20241223150907.1591-1-ryncsn@gmail.com Fixes:fb56fdf8b9("mm/list_lru: split the lock to per-cgroup scope") Signed-off-by: Kairui Song <kasong@tencent.com> Closes: https://lore.kernel.org/lkml/Z2Bz9t92Be9l1xqj@lappy/ Cc: Chengming Zhou <zhouchengming@bytedance.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Matthew Wilcox <willy@infradead.org> Cc: Michal Hocko <mhocko@suse.com> Cc: Muchun Song <muchun.song@linux.dev> Cc: Qi Zheng <zhengqi.arch@bytedance.com> Cc: Roman Gushchin <roman.gushchin@linux.dev> Cc: Sasha Levin <sashal@kernel.org> Cc: Shakeel Butt <shakeel.butt@linux.dev> Cc: Yu Zhao <yuzhao@google.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.