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 Restructured Text 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.
Muchun Song
authored
When pages are swapped in, the VM may retain the swap copy to avoid repeated writes in the future. It's also retained if shared pages are faulted back in some processes, but not in others. During that time we have an in-memory copy of the page, as well as an on-swap copy. Cgroup1 and cgroup2 handle these overlapping lifetimes slightly differently due to the nature of how they account memory and swap: Cgroup1 has a unified memory+swap counter that tracks a data page regardless whether it's in-core or swapped out. On swapin, we transfer the charge from the swap entry to the newly allocated swapcache page, even though the swap entry might stick around for a while. That's why we have a mem_cgroup_uncharge_swap() call inside mem_cgroup_charge(). Cgroup2 tracks memory and swap as separate, independent resources and thus has split memory and swap counters. On swapin, we charge the newly allocated swapcache page as memory, while the swap slot in turn must remain charged to the swap counter as long as its allocated too. The cgroup2 logic was broken by commit 2d1c4980 ("mm: memcontrol: make swap tracking an integral part of memory control"), because it accidentally removed the do_memsw_account() check in the branch inside mem_cgroup_uncharge() that was supposed to tell the difference between the charge transfer in cgroup1 and the separate counters in cgroup2. As a result, cgroup2 currently undercounts retained swap to varying degrees: swap slots are cached up to 50% of the configured limit or total available swap space; partially faulted back shared pages are only limited by physical capacity. This in turn allows cgroups to significantly overconsume their alloted swap space. Add the do_memsw_account() check back to fix this problem. Link: https://lkml.kernel.org/r/20210217153237.92484-1-songmuchun@bytedance.com Fixes: 2d1c4980 ("mm: memcontrol: make swap tracking an integral part of memory control") Signed-off-by:Muchun Song <songmuchun@bytedance.com> Acked-by:
Johannes Weiner <hannes@cmpxchg.org> Reviewed-by:
Shakeel Butt <shakeelb@google.com> Acked-by:
Michal Hocko <mhocko@suse.com> Cc: Vladimir Davydov <vdavydov.dev@gmail.com> Cc: <stable@vger.kernel.org> [5.8+] Signed-off-by:
Andrew Morton <akpm@linux-foundation.org> Signed-off-by:
Linus Torvalds <torvalds@linux-foundation.org>
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