73bc32875e
Within try_to_unmap_one(), page_vma_mapped_walk() races with other PTE modifications preceded by pte clear. While iterating over PTEs of a large folio, it only starts acquiring PTL from the first valid (present) PTE. PTE modifications can temporarily set PTEs to pte_none. Consequently, the initial PTEs of a large folio might be skipped in try_to_unmap_one(). For example, for an anon folio, if we skip PTE0, we may have PTE0 which is still present, while PTE1 ~ PTE(nr_pages - 1) are swap entries after try_to_unmap_one(). So folio will be still mapped, the folio fails to be reclaimed and is put back to LRU in this round. This also breaks up PTEs optimization such as CONT-PTE on this large folio and may lead to accident folio_split() afterwards. And since a part of PTEs are now swap entries, accessing those parts will introduce overhead - do_swap_page. Although the kernel can withstand all of the above issues, the situation still seems quite awkward and warrants making it more ideal. The same race also occurs with small folios, but they have only one PTE, thus, it won't be possible for them to be partially unmapped. This patch holds PTL from PTE0, allowing us to avoid reading PTE values that are in the process of being transformed. With stable PTE values, we can ensure that this large folio is either completely reclaimed or that all PTEs remain untouched in this round. A corner case is that if we hold PTL from PTE0 and most initial PTEs have been really unmapped before that, we may increase the duration of holding PTL. Thus we only apply this optimization to folios which are still entirely mapped (not in deferred_split list). [akpm@linux-foundation.org: rewrap comment, per Matthew] Link: https://lkml.kernel.org/r/20240306095219.71086-1-21cnbao@gmail.com Signed-off-by: Barry Song <v-songbaohua@oppo.com> Acked-by: David Hildenbrand <david@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: Chris Li <chrisl@kernel.org> Cc: Chuanhua Han <hanchuanhua@oppo.com> Cc: Gao Xiang <xiang@kernel.org> Cc: Huang, Ying <ying.huang@intel.com> Cc: Hugh Dickins <hughd@google.com> Cc: Kefeng Wang <wangkefeng.wang@huawei.com> Cc: Matthew Wilcox (Oracle) <willy@infradead.org> Cc: Michal Hocko <mhocko@suse.com> Cc: Ryan Roberts <ryan.roberts@arm.com> Cc: Yang Shi <shy828301@gmail.com> 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.