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SubjectRe: kernel BUG at mm/highmem.c:417! invalid opcode: 0000 EIP: zero_user_segments
On Tue, Nov 24, 2020 at 06:16:28PM +0100, Sebastian Andrzej Siewior wrote:
> On 2020-11-24 18:52:44 [+0530], Naresh Kamboju wrote:
> > While running LTP test case access01 the following kernel BUG
> > noticed on linux next 20201124 tag kernel on i386.
> >
> > git short log:
> > ----------------
> > git log --oneline next-20201120..next-20201124 -- mm/highmem.c
> > d9927d46febf Merge branch 'akpm-current/current'
> > 72d22a0d0e86 mm: support THPs in zero_user_segments
> > 2a656cad337e mm/highmem: Take kmap_high_get() properly into account
> >
> > Please find these easy steps to reproduce the kernel build and boot.
>
> This BUG_ON() is in zero_user_segments() which ash been added in commit
> 72d22a0d0e86 mm: support THPs in zero_user_segments
>
> > [ 50.852189] kernel BUG at mm/highmem.c:417!
>
> I managed to capture one invocation with:
> zero_user_segments(0xd4367a90,
> 0x1000, 0x1000,
> 0x0, 0x50)
> page_compound() -> 1
> page_size() -> 4096

Thanks for debugging this! I didn't realise start1 was allowed to be
less than start2. Try this ... (systemd is sabotaging my efforts to
test an i386 kernel)

diff --git a/mm/highmem.c b/mm/highmem.c
index 3e1087f2b735..6306a535dd9c 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -369,46 +369,39 @@ void zero_user_segments(struct page *page, unsigned start1, unsigned end1,
BUG_ON(end1 > page_size(page) || end2 > page_size(page));

for (i = 0; i < compound_nr(page); i++) {
- void *kaddr;
- unsigned this_end;
+ void *kaddr = NULL;

- if (end1 == 0 && start2 >= PAGE_SIZE) {
- start2 -= PAGE_SIZE;
- end2 -= PAGE_SIZE;
- continue;
- }
+ if (start1 < PAGE_SIZE || start2 < PAGE_SIZE)
+ kaddr = kmap_atomic(page + i);

if (start1 >= PAGE_SIZE) {
start1 -= PAGE_SIZE;
end1 -= PAGE_SIZE;
- if (start2) {
- start2 -= PAGE_SIZE;
- end2 -= PAGE_SIZE;
- }
- continue;
- }
-
- kaddr = kmap_atomic(page + i);
+ } else {
+ unsigned this_end = min_t(unsigned, end1, PAGE_SIZE);

- this_end = min_t(unsigned, end1, PAGE_SIZE);
- if (end1 > start1)
- memset(kaddr + start1, 0, this_end - start1);
- end1 -= this_end;
- start1 = 0;
+ if (end1 > start1)
+ memset(kaddr + start1, 0, this_end - start1);
+ end1 -= this_end;
+ start1 = 0;
+ }

if (start2 >= PAGE_SIZE) {
start2 -= PAGE_SIZE;
end2 -= PAGE_SIZE;
} else {
- this_end = min_t(unsigned, end2, PAGE_SIZE);
+ unsigned this_end = min_t(unsigned, end2, PAGE_SIZE);
+
if (end2 > start2)
memset(kaddr + start2, 0, this_end - start2);
end2 -= this_end;
start2 = 0;
}

- kunmap_atomic(kaddr);
- flush_dcache_page(page + i);
+ if (kaddr) {
+ kunmap_atomic(kaddr);
+ flush_dcache_page(page + i);
+ }

if (!end1 && !end2)
break;
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