On Tue, Jul 8, 2025 at 8:57 AM Sasha Levin sashal@kernel.org wrote:
On Tue, Jul 08, 2025 at 08:39:47AM -0700, Suren Baghdasaryan wrote:
On Tue, Jul 8, 2025 at 8:33 AM Sasha Levin sashal@kernel.org wrote:
On Tue, Jul 08, 2025 at 05:10:44PM +0200, David Hildenbrand wrote:
On 01.07.25 02:57, Andrew Morton wrote:
On Sun, 29 Jun 2025 23:19:58 -0400 Sasha Levin sashal@kernel.org wrote:
When handling non-swap entries in move_pages_pte(), the error handling for entries that are NOT migration entries fails to unmap the page table entries before jumping to the error handling label.
This results in a kmap/kunmap imbalance which on CONFIG_HIGHPTE systems triggers a WARNING in kunmap_local_indexed() because the kmap stack is corrupted.
Example call trace on ARM32 (CONFIG_HIGHPTE enabled): WARNING: CPU: 1 PID: 633 at mm/highmem.c:622 kunmap_local_indexed+0x178/0x17c Call trace: kunmap_local_indexed from move_pages+0x964/0x19f4 move_pages from userfaultfd_ioctl+0x129c/0x2144 userfaultfd_ioctl from sys_ioctl+0x558/0xd24
The issue was introduced with the UFFDIO_MOVE feature but became more frequent with the addition of guard pages (commit 7c53dfbdb024 ("mm: add PTE_MARKER_GUARD PTE marker")) which made the non-migration entry code path more commonly executed during userfaultfd operations.
Fix this by ensuring PTEs are properly unmapped in all non-swap entry paths before jumping to the error handling label, not just for migration entries.
I don't get it.
--- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1384,14 +1384,15 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, entry = pte_to_swp_entry(orig_src_pte); if (non_swap_entry(entry)) {
pte_unmap(src_pte);pte_unmap(dst_pte);src_pte = dst_pte = NULL; if (is_migration_entry(entry)) {
pte_unmap(src_pte);pte_unmap(dst_pte);src_pte = dst_pte = NULL; migration_entry_wait(mm, src_pmd, src_addr); err = -EAGAIN;} else
} else { err = -EFAULT;} goto out;where we have
out: ... if (dst_pte) pte_unmap(dst_pte); if (src_pte) pte_unmap(src_pte);
AI slop?
Nah, this one is sadly all me :(
I was trying to resolve some of the issues found with linus-next on LKFT, and misunderstood the code. Funny enough, I thought that the change above "fixed" it by making the warnings go away, but clearly is the wrong thing to do so I went back to the drawing table...
If you're curious, here's the issue: https://qa-reports.linaro.org/lkft/sashal-linus-next/build/v6.13-rc7-43418-g...
Any way to symbolize that Call trace? I can't find build artefacts to extract vmlinux image...
The build artifacts are at https://storage.tuxsuite.com/public/linaro/lkft/builds/2zSrTao2x4P640QKIx18J... but I couldn't get it to do the right thing. I'm guessing that I need some magical arm32 toolchain bits that I don't carry:
cat tr.txt | ./scripts/decode_stacktrace.sh vmlinux <4>[ 38.566145] ------------[ cut here ]------------ <4>[ 38.566392] WARNING: CPU: 1 PID: 637 at mm/highmem.c:622 kunmap_local_indexed+0x198/0x1a4 <4>[ 38.569398] Modules linked in: nfnetlink ip_tables x_tables <4>[ 38.570481] CPU: 1 UID: 0 PID: 637 Comm: uffd-unit-tests Not tainted 6.16.0-rc4 #1 NONE <4>[ 38.570815] Hardware name: Generic DT based system <4>[ 38.571073] Call trace: <4>[ 38.571239] unwind_backtrace from show_stack (arch/arm64/kernel/stacktrace.c:465) <4>[ 38.571602] show_stack from dump_stack_lvl (lib/dump_stack.c:118 (discriminator 1)) <4>[ 38.571805] dump_stack_lvl from __warn (kernel/panic.c:791) <4>[ 38.572002] __warn from warn_slowpath_fmt+0xa8/0x174 <4>[ 38.572290] warn_slowpath_fmt from kunmap_local_indexed+0x198/0x1a4 <4>[ 38.572520] kunmap_local_indexed from move_pages_pte+0xc40/0xf48 <4>[ 38.572970] move_pages_pte from move_pages+0x428/0x5bc <4>[ 38.573189] move_pages from userfaultfd_ioctl+0x900/0x1ec0 <4>[ 38.573376] userfaultfd_ioctl from sys_ioctl+0xd24/0xd90 <4>[ 38.573581] sys_ioctl from ret_fast_syscall+0x0/0x5c <4>[ 38.573810] Exception stack(0xf9d69fa8 to 0xf9d69ff0) <4>[ 38.574546] 9fa0: 00001000 00000005 00000005 c028aa05 b2d3ecd8 b2d3ecc8 <4>[ 38.574919] 9fc0: 00001000 00000005 b2d3ece0 00000036 b2d3ed84 b2d3ed50 b2d3ed7c b2d3ed58 <4>[ 38.575131] 9fe0: 00000036 b2d3ecb0 b6df1861 b6d5f736 <4>[ 38.575511] ---[ end trace 0000000000000000 ]---
Ah, I know what's going on. 6.13.rc7 which is used in this test does not have my fix 927e926d72d9 ("userfaultfd: fix PTE unmapping stack-allocated PTE copies") (see https://elixir.bootlin.com/linux/v6.13.7/source/mm/userfaultfd.c#L1284). It was backported into 6.13.rc8. So, it tries to unmap a copy of a mapped PTE, which will fail when CONFIG_HIGHPTE is enabled. So, it makes sense that it is failing on arm32.
-- Thanks, Sasha