在 2022/4/18 17:28, Anshuman Khandual 写道:
On 4/18/22 09:14, Tong Tiangen wrote:
From: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
[...]
#endif
Ran this series on arm64 platform after enabling
- CONFIG_PAGE_TABLE_CHECK
- CONFIG_PAGE_TABLE_CHECK_ENFORCED (avoiding kernel command line option)
After some time, the following error came up
[ 23.266013] ------------[ cut here ]------------
[ 23.266807] kernel BUG at mm/page_table_check.c:90!
[ 23.267609] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[ 23.268503] Modules linked in:
[ 23.269012] CPU: 1 PID: 30 Comm: khugepaged Not tainted 5.18.0-rc3-00004-g60aa8e363a91 #2
[ 23.270383] Hardware name: linux,dummy-virt (DT)
[ 23.271210] pstate: 40400005 (nZcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 23.272445] pc : page_table_check_clear.isra.6+0x114/0x148
[ 23.273429] lr : page_table_check_clear.isra.6+0x64/0x148
[ 23.274395] sp : ffff80000afb3ca0
[ 23.274994] x29: ffff80000afb3ca0 x28: fffffc00022558e8 x27: ffff80000a27f628
[ 23.276260] x26: ffff800009f9f2b0 x25: ffff00008a8d5000 x24: ffff800009f09fa0
[ 23.277527] x23: 0000ffff89e00000 x22: ffff800009f09fb8 x21: ffff000089414cc0
[ 23.278798] x20: 0000000000000200 x19: fffffc00022a0000 x18: 0000000000000001
[ 23.280066] x17: 0000000000000001 x16: 0000000000000000 x15: 0000000000000003
[ 23.281331] x14: 0000000000000068 x13: 00000000000000c0 x12: 0000000000000010
[ 23.282602] x11: fffffc0002320008 x10: fffffc0002320000 x9 : ffff800009fa1000
[ 23.283868] x8 : 00000000ffffffff x7 : 0000000000000001 x6 : ffff800009fa1f08
[ 23.285135] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
[ 23.286406] x2 : 00000000ffffffff x1 : ffff000080f2800c x0 : ffff000080f28000
[ 23.287673] Call trace:
[ 23.288123] page_table_check_clear.isra.6+0x114/0x148
[ 23.289043] __page_table_check_pmd_clear+0x3c/0x50
[ 23.289918] pmdp_collapse_flush+0x114/0x370
[ 23.290692] khugepaged+0x1170/0x19e0
[ 23.291356] kthread+0x110/0x120
[ 23.291945] ret_from_fork+0x10/0x20
[ 23.292596] Code: 91001041 b8e80024 51000482 36fffd62 (d4210000)
[ 23.293678] ---[ end trace 0000000000000000 ]---
[ 23.294511] note: khugepaged[30] exited with preempt_count 2
Looking into file mm/page_table_check.c where this problem occured.
/*
* An enty is removed from the page table, decrement the counters for that page
* verify that it is of correct type and counters do not become negative.
*/
static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
unsigned long pfn, unsigned long pgcnt)
{
struct page_ext *page_ext;
struct page *page;
unsigned long i;
bool anon;
if (!pfn_valid(pfn))
return;
page = pfn_to_page(pfn);
page_ext = lookup_page_ext(page);
anon = PageAnon(page);
for (i = 0; i < pgcnt; i++) {
struct page_table_check *ptc = get_page_table_check(page_ext);
if (anon) {
BUG_ON(atomic_read(&ptc->file_map_count));
BUG_ON(atomic_dec_return(&ptc->anon_map_count) < 0);
} else {
BUG_ON(atomic_read(&ptc->anon_map_count));
Triggered here ====>> BUG_ON(atomic_dec_return(&ptc->file_map_count) < 0);
}
page_ext = page_ext_next(page_ext);
}
}
Could you explain what was expected during pmdp_collapse_flush() which when
failed, triggered this BUG_ON() ? This counter seems to be page table check
specific, could it just go wrong ? I have not looked into the details about
page table check mechanism.
- Anshuman
.
Hi Anshuman:
Thanks for your job.
Let me briefly explain the principle of page table check(PTC).
PTC introduces the following struct for page mapping type count:
struct page_table_check {
atomic_t anon_map_count;
atomic_t file_map_count;
};
This structure can be obtained by "lookup_page_ext(page)"
When page table entries are set(pud/pmd/pte), page_table_check_set() is
called to increase the page mapping count, Also check for errors (eg:if
a page is used for anonymous mapping, then the page cannot be used for
file mapping at the same time).
When page table entries are clear(pud/pmd/pte), page_table_check_clear()
is called to decrease the page mapping count, Also check for errors.
The error check rules are described in the following documents:
Documentation/vm/page_table_check.rst
The setting and clearing of page table entries are symmetrical.
Here __page_table_check_pmd_clear() trigger BUGON which indicates that
the pmd entry file mapping count has become negative.
I guess if PTC didn't detect this exception, would there have been any
problems?
Thanks,
Tong.
