[BUG] mm/ksm: KSM merge clears soft-dirty tracking

[shaos <shawnq1@163.com>]

Hello,

KSM can clear the soft-dirty state of a page when it replaces a PTE with a shared KSM page. After clearing soft-dirty state through /proc/self/clear_refs and writing to two identical MADV_MERGEABLE pages, both pagemap entries show bit 55 set. Once KSM merges them and their PFNs become equal, one entry has bit 55 cleared even though that mapping was written since the last clear_refs operation.

replace_page() builds the new PTE with mk_pte(kpage, vma->vm_page_prot) and does not preserve pte_soft_dirty(orig_pte). The marker loss can make userspace dirty-page trackers miss a page modified since their last checkpoint. The PoC makes both pages dirty before merging, so the result does not depend on which page KSM keeps as the stable page.

The same replacement path also loses userfaultfd RWP markers; that separate observable is documented at https://gist.github.com/shaos26/e887665d7868dc323d73a1d678cacd3d. This report focuses on soft-dirty tracking.

HEAD commit:    24a19b953e2a3d0ea5a8c4951a0ab8f32720c907
source version: Linux 7.3.0-rc7
architecture:   x86-64
compiler:       gcc 11.4.0, GNU ld 2.38
QEMU:           6.2.0

console output: https://gist.githubusercontent.com/shaos26/9db36768f01eb83c6aa14ee9fc057388/raw/59077135f035791493d6d487c9e619c16380518f/console-output.txt
kernel config: https://gist.githubusercontent.com/shaos26/9db36768f01eb83c6aa14ee9fc057388/raw/baa5ee61ce748667104c480560ab5116a100e3fd/kernel-config.txt
reproducer: https://gist.githubusercontent.com/shaos26/9db36768f01eb83c6aa14ee9fc057388/raw/06005564982eab5cbcec26c5d62ad86c7261a155/poc.c

Build the kernel from the revision above with the attached config. Build the static PoC with:

    gcc -static -O2 -Wall -Wextra -Werror -o poc poc.c

Place it at /init in an initramfs and boot a fresh QEMU guest:

    timeout 300 qemu-system-x86_64 -machine pc -cpu max -smp 1 -m 2048 \
        -kernel build/arch/x86/boot/bzImage -initrd initramfs.cpio.gz \
        -append "console=ttyS0,115200 rdinit=/init panic=-1 oops=panic loglevel=7 vivid.n_devs=1 dummy_hcd.num=1 netrom.nr_ndevs=1 rose.rose_ndevs=1 max_loop=1 comedi.comedi_num_legacy_minors=0" \
        -display none -serial stdio -monitor none -no-reboot

Two positive boots reported RESULT=KSM_SOFTDIRTY_LOSS_CONFIRMED with equal PFNs and one soft-dirty bit cleared. The non-merging control kept distinct PFNs and both bits set. No privilege escalation or memory corruption is claimed. The affected KSM replacement logic appears unchanged in upstream master f47d223627f8; runtime validation was on v7.3-rc7.

Thanks,

shaos