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cd197c3a20
In a Copy-on-Write (CoW) scenario, the last subpage will reuse the entire
large folio, resulting in the waste of (nr_pages - 1) pages. This wasted
memory remains allocated until it is either unmapped or memory reclamation
occurs.
The following small program can serve as evidence of this behavior
main()
{
#define SIZE 1024 * 1024 * 1024UL
void *p = malloc(SIZE);
memset(p, 0x11, SIZE);
if (fork() == 0)
_exit(0);
memset(p, 0x12, SIZE);
printf("done\n");
while(1);
}
For example, using a 1024KiB mTHP by:
echo always > /sys/kernel/mm/transparent_hugepage/hugepages-1024kB/enabled
(1) w/o the patch, it takes 2GiB,
Before running the test program,
/ # free -m
total used free shared buff/cache available
Mem: 5754 84 5692 0 17 5669
Swap: 0 0 0
/ # /a.out &
/ # done
After running the test program,
/ # free -m
total used free shared buff/cache available
Mem: 5754 2149 3627 0 19 3605
Swap: 0 0 0
(2) w/ the patch, it takes 1GiB only,
Before running the test program,
/ # free -m
total used free shared buff/cache available
Mem: 5754 89 5687 0 17 5664
Swap: 0 0 0
/ # /a.out &
/ # done
After running the test program,
/ # free -m
total used free shared buff/cache available
Mem: 5754 1122 4655 0 17 4632
Swap: 0 0 0
This patch migrates the last subpage to a small folio and immediately
returns the large folio to the system. It benefits both memory availability
and anti-fragmentation.
Link: https://lkml.kernel.org/r/20240308092721.144735-1-21cnbao@gmail.com
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Lance Yang <ioworker0@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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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.