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

[Peter Xu]

On Thu, Oct 06, 2022 at 11:29:29AM +0200, David Hildenbrand wrote:
> On 05.10.22 22:35, Peter Xu wrote:
> > 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?
> 
> Yes, I can give it a try. What I dislike about ksm_test is that it's a
> mixture of benchmarks and test cases that have to explicitly triggered by
> parameters. It's not a simple "run all available test cases" tests as we
> know it. So maybe something separate (or having it as part of the uffd
> tests) makes more sense.

We can add an entry into run_vmtests.sh.  That's also what current ksm_test
does.

Yes adding into uffd test would work too, but I do have a plan that we
should move functional tests out of userfaultfd.c, leaving that with the
stress test only.  Not really a big deal, though.

> 
> > 
> > > 
> > > 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.
> 
> A fix without FAULT_FLAG_UNSHARE is not straight forward. We really don't
> want to notify user space about write events here (because there is none).
> And there is no way around the uffd handling in WP code without that.
> 
> FAULT_FLAG_ALLOW_RETRY would rely on userfaultfd triggering and having to
> resolve the WP event.

Right it'll be very much a false positive, but the userspace should be fine
with it e.g. for live snapshot we need to copy page earlier; it still won't
stop the process from running along the way.  But I agree that's not ideal.

-- 
Peter Xu