Re: [PATCH v2] mm/ksm: keep soft-dirty and uffd-wp when merging a page

[Mohammed EL Kadiri]

This works for me, on b306deee4cba (mm-testing), x86_64,
CONFIG_KSM=y, CONFIG_USERFAULTFD=y, CONFIG_DEBUG_VM=y, KASAN enabled,
THP disabled.

Reproducer: merge 64 identical anon pages via KSM, arm synchronous
userfaultfd-wp on one page, then MADV_UNMERGEABLE it without touching
it. The page should come out private with WP semantics intact.

On the same commit, before applying your wp_page_copy()/handle_pte_fault()
fix:

  UNMERGE ret=0 errno=0
  AFTER_UNMERGE target_pfn=1153393 peer_pfn=1153393 target_KPF_KSM=1
  FAIL: MADV_UNMERGEABLE returned success but target remains shared/KSM

and every run also hits this in dmesg:

  FAULT_FLAG_ALLOW_RETRY missing c80
  Call Trace:
   handle_userfault.cold+0x30/0x3f
   __handle_mm_fault+0xde9/0x1c30
   handle_mm_fault+0x13b/0x440
   break_ksm+0x8a/0x100
   ksm_madvise+0x467/0x780
   madvise_vma_behavior+0x1d1/0x1e40
   madvise_do_behavior+0x431/0x14b0
   do_madvise+0x162/0x210

break_ksm()'s internal fault-in to unshare the page is landing in the
uffd-wp fault handler, which isn't expecting it. 8/8 runs across two
boots, same trace every time.

After applying the fix:

  AFTER_UNMERGE target_pfn=1063999 peer_pfn=1064929 target_KPF_KSM=0
  PASS: target is private and no longer KSM-backed
  FAULT event=18 flags=0x8 address=0x7f3dcb727000
  PASS: read generated RWP fault and remained blocked

No warnings or call traces in dmesg. 8/8 runs across two boots.

Tested-by: Mohammed EL Kadiri <med08elkadiri@xxxxxxxxx>