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Bijan Tabatabai 4bae7111bf mm: consider non-anon swap cache folios in folio_expected_ref_count() 
commit f183663901 upstream.

Currently, folio_expected_ref_count() only adds references for the swap
cache if the folio is anonymous.  However, according to the comment above
the definition of PG_swapcache in enum pageflags, shmem folios can also
have PG_swapcache set.  This patch makes sure references for the swap
cache are added if folio_test_swapcache(folio) is true.

This issue was found when trying to hot-unplug memory in a QEMU/KVM
virtual machine.  When initiating hot-unplug when most of the guest memory
is allocated, hot-unplug hangs partway through removal due to migration
failures.  The following message would be printed several times, and would
be printed again about every five seconds:

[   49.641309] migrating pfn b12f25 failed ret:7
[   49.641310] page: refcount:2 mapcount:0 mapping:0000000033bd8fe2 index:0x7f404d925 pfn:0xb12f25
[   49.641311] aops:swap_aops
[   49.641313] flags: 0x300000000030508(uptodate|active|owner_priv_1|reclaim|swapbacked|node=0|zone=3)
[   49.641314] raw: 0300000000030508 ffffed312c4bc908 ffffed312c4bc9c8 0000000000000000
[   49.641315] raw: 00000007f404d925 00000000000c823b 00000002ffffffff 0000000000000000
[   49.641315] page dumped because: migration failure

When debugging this, I found that these migration failures were due to
__migrate_folio() returning -EAGAIN for a small set of folios because the
expected reference count it calculates via folio_expected_ref_count() is
one less than the actual reference count of the folios.  Furthermore, all
of the affected folios were not anonymous, but had the PG_swapcache flag
set, inspiring this patch.  After applying this patch, the memory
hot-unplug behaves as expected.

I tested this on a machine running Ubuntu 24.04 with kernel version
6.8.0-90-generic and 64GB of memory.  The guest VM is managed by libvirt
and runs Ubuntu 24.04 with kernel version 6.18 (though the head of the
mm-unstable branch as a Dec 16, 2025 was also tested and behaves the same)
and 48GB of memory.  The libvirt XML definition for the VM can be found at
[1].  CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_MOVABLE is set in the guest
kernel so the hot-pluggable memory is automatically onlined.

Below are the steps to reproduce this behavior:

1) Define and start and virtual machine
  host$ virsh -c qemu:///system define ./test_vm.xml # test_vm.xml from [1]
  host$ virsh -c qemu:///system start test_vm

2) Setup swap in the guest
  guest$ sudo fallocate -l 32G /swapfile
  guest$ sudo chmod 0600 /swapfile
  guest$ sudo mkswap /swapfile
  guest$ sudo swapon /swapfile

3) Use alloc_data [2] to allocate most of the remaining guest memory
  guest$ ./alloc_data 45

4) In a separate guest terminal, monitor the amount of used memory
  guest$ watch -n1 free -h

5) When alloc_data has finished allocating, initiate the memory
hot-unplug using the provided xml file [3]
  host$ virsh -c qemu:///system detach-device test_vm ./remove.xml --live

After initiating the memory hot-unplug, you should see the amount of
available memory in the guest decrease, and the amount of used swap data
increase.  If everything works as expected, when all of the memory is
unplugged, there should be around 8.5-9GB of data in swap.  If the
unplugging is unsuccessful, the amount of used swap data will settle below
that.  If that happens, you should be able to see log messages in dmesg
similar to the one posted above.

Link: https://lkml.kernel.org/r/20251216200727.2360228-1-bijan311@gmail.com
Link: https://github.com/BijanT/linux_patch_files/blob/main/test_vm.xml [1]
Link: https://github.com/BijanT/linux_patch_files/blob/main/alloc_data.c [2]
Link: https://github.com/BijanT/linux_patch_files/blob/main/remove.xml [3]
Fixes: 86ebd50224 ("mm: add folio_expected_ref_count() for reference count calculation")
Signed-off-by: Bijan Tabatabai <bijan311@gmail.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shivank Garg <shivankg@amd.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2026-01-08 10:17:11 +01:00
arch LoongArch: Use unsigned long for _end and _text 2026-01-08 10:17:06 +01:00
block block: Clear BLK_ZONE_WPLUG_PLUGGED when aborting plugged BIOs 2026-01-08 10:16:58 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: seqiv - Do not use req->iv after crypto_aead_encrypt 2026-01-08 10:16:43 +01:00
Documentation dt-bindings: PCI: qcom,pcie-sm8550: Add missing required power-domains and resets 2026-01-02 12:57:27 +01:00
drivers pmdomain: imx: Fix reference count leak in imx_gpc_probe() 2026-01-08 10:17:09 +01:00
fs lockd: fix vfs_test_lock() calls 2026-01-08 10:17:10 +01:00
include mm: consider non-anon swap cache folios in folio_expected_ref_count() 2026-01-08 10:17:11 +01:00
init printk changes for 6.18 2025-10-04 11:13:11 -07:00
io_uring io_uring/rsrc: fix lost entries after cloned range 2026-01-02 12:57:30 +01:00
ipc namespace-6.18-rc1 2025-09-29 11:20:29 -07:00
kernel kernel/kexec: fix IMA when allocation happens in CMA area 2026-01-08 10:17:10 +01:00
lib idr: fix idr_alloc() returning an ID out of range 2026-01-08 10:17:10 +01:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm mm/page_owner: fix memory leak in page_owner_stack_fops->release() 2026-01-08 10:17:10 +01:00
net mptcp: fallback earlier on simult connection 2026-01-08 10:17:09 +01:00
rust rust: io: add typedef for phys_addr_t 2026-01-02 12:57:05 +01:00
samples samples: rust: fix endianness issue in rust_driver_pci 2026-01-02 12:57:05 +01:00
scripts mcb: Add missing modpost build support 2026-01-08 10:16:47 +01:00
security kernel/kexec: change the prototype of kimage_map_segment() 2026-01-08 10:17:10 +01:00
sound ASoC: qcom: qdsp6: q6asm-dai: set 10 ms period and buffer alignment. 2026-01-08 10:16:54 +01:00
tools idr: fix idr_alloc() returning an ID out of range 2026-01-08 10:17:10 +01:00
usr gen_init_cpio: Ignore fsync() returning EINVAL on pipes 2025-10-07 09:53:05 -07:00
virt KVM: Fix last_boosted_vcpu index assignment bug 2026-01-02 12:57:21 +01:00
.clang-format memblock: drop for_each_free_mem_pfn_range_in_zone_from() 2025-09-14 08:49:03 +03:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore .gitignore: ignore compile_commands.json globally 2025-08-12 15:53:55 -07:00
.mailmap 8 hotfixes. 4 are cc:stable, 7 are against mm/. 2025-11-26 12:38:05 -08:00
.pylintrc tools: docs: parse-headers.py: move it from sphinx dir 2025-08-29 15:54:42 -06:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: mark ISDN subsystem as orphan 2025-10-27 17:49:45 -07:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS soc: fixes for 6.18, part 4 2025-11-28 09:57:31 -08:00
Makefile Linux 6.18.3 2026-01-02 12:57:31 +01:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

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.