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@xxxxxxxxxx>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
--
Peter Xu
