Re: [PATCH v1 4/7] mm/ksm: fix KSM COW breaking with userfaultfd-wp via FAULT_FLAG_UNSHARE

[Peter Xu <peterx@redhat.com>]

On Fri, Sep 30, 2022 at 04:19:28PM +0200, David Hildenbrand wrote:
> Let's stop breaking COW via a fake write fault and let's use
> FAULT_FLAG_UNSHARE instead. This avoids any wrong side effects of the fake
> write fault, such as mapping the PTE writable and marking the pte
> dirty/softdirty.
> 
> Also, this fixes KSM interaction with userfaultfd-wp: when we have a KSM
> page that's write-protected by userfaultfd, break_ksm()->handle_mm_fault()
> will fail with VM_FAULT_SIGBUS and will simpy return in break_ksm() with 0.
> The warning in dmesg indicates this wrong handling:
> 
> [  230.096368] FAULT_FLAG_ALLOW_RETRY missing 881
> [  230.100822] CPU: 1 PID: 1643 Comm: ksm-uffd-wp [...]
> [  230.110124] Hardware name: [...]
> [  230.117775] Call Trace:
> [  230.120227]  <TASK>
> [  230.122334]  dump_stack_lvl+0x44/0x5c
> [  230.126010]  handle_userfault.cold+0x14/0x19
> [  230.130281]  ? tlb_finish_mmu+0x65/0x170
> [  230.134207]  ? uffd_wp_range+0x65/0xa0
> [  230.137959]  ? _raw_spin_unlock+0x15/0x30
> [  230.141972]  ? do_wp_page+0x50/0x590
> [  230.145551]  __handle_mm_fault+0x9f5/0xf50
> [  230.149652]  ? mmput+0x1f/0x40
> [  230.152712]  handle_mm_fault+0xb9/0x2a0
> [  230.156550]  break_ksm+0x141/0x180
> [  230.159964]  unmerge_ksm_pages+0x60/0x90
> [  230.163890]  ksm_madvise+0x3c/0xb0
> [  230.167295]  do_madvise.part.0+0x10c/0xeb0
> [  230.171396]  ? do_syscall_64+0x67/0x80
> [  230.175157]  __x64_sys_madvise+0x5a/0x70
> [  230.179082]  do_syscall_64+0x58/0x80
> [  230.182661]  ? do_syscall_64+0x67/0x80
> [  230.186413]  entry_SYSCALL_64_after_hwframe+0x63/0xcd

Since it's already there, worth adding the test into ksm_test.c?

> 
> Consequently, we will no longer trigger a fake write fault and break COW
> without any such side-effects.
> 
> This is primarily a fix for KSM+userfaultfd-wp, however, the fake write
> fault was always questionable. As this fix is not easy to backport and it's
> not very critical, let's not cc stable.

A patch to cc most of the stable would probably need to still go with the
old write approach but attaching ALLOW_RETRY.  But I agree maybe that may
not need to bother, or a report should have arrived earlier..  The unshare
approach looks much cleaner indeed.

> 
> Fixes: 529b930b87d9 ("userfaultfd: wp: hook userfault handler to write protection fault")
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu