cc864ebba5
The purpose is stopping splitting large folios whose mapcount are 2 or
above. Folios whose estimated_shares = 0 should be still perfect and even
better candidates than estimated_shares = 1.
Consider a pte-mapped large folio with 16 subpages, if we unmap 1-15, the
current code will split folios and reclaim them while madvise goes on this
folio; but if we unmap subpage 0, we will keep this folio and break. This
is weird.
For pmd-mapped large folios, we can still use "= 1" as the condition as
anyway we have the entire map for it. So this patch doesn't change the
condition for pmd-mapped large folios. This also explains why we had been
using "= 1" for both pmd-mapped and pte-mapped large folios before commit
07e8c82b5eff ("madvise: convert madvise_cold_or_pageout_pte_range() to use
folios"), because in the past, we used the mapcount of the specific
subpage, since the subpage had pte present, its mapcount wouldn't be 0.
The problem can be quite easily reproduced by writing a small program,
unmapping the first subpage of a pte-mapped large folio vs. unmapping
anyone other than the first subpage.
Link: https://lkml.kernel.org/r/20240221085036.105621-1-21cnbao@gmail.com
Fixes: 2f406263e3e9 ("madvise:madvise_cold_or_pageout_pte_range(): don't use mapcount() against large folio for sharing check")
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Yin Fengwei <fengwei.yin@intel.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Yang Shi <shy828301@gmail.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 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.