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

From: David Hildenbrand
Date: Thu Oct 06 2022 - 05:29:49 EST

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

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.

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.

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

Acked-by: Peter Xu <peterx@xxxxxxxxxx>



David / dhildenb