khugepaged might_sleep() warn due to CONFIG_HIGHPTE=y

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

Hi,

I got some khugepaged spew on a 32bit x86:

[  217.490026] BUG: sleeping function called from invalid context at include/linux/mmu_notifier.h:346
[  217.492826] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 25, name: khugepaged
[  217.495589] INFO: lockdep is turned off.
[  217.498371] CPU: 1 PID: 25 Comm: khugepaged Not tainted 5.4.0-rc5-elk+ #206
[  217.501233] Hardware name: System manufacturer P5Q-EM/P5Q-EM, BIOS 2203    07/08/2009
[  217.501697] Call Trace:
[  217.501697]  dump_stack+0x66/0x8e
[  217.501697]  ___might_sleep.cold.96+0x95/0xa6
[  217.501697]  __might_sleep+0x2e/0x80
[  217.501697]  collapse_huge_page.isra.51+0x5ac/0x1360
[  217.501697]  ? __alloc_pages_nodemask+0xec/0xf80
[  217.501697]  ? __alloc_pages_nodemask+0x191/0xf80
[  217.501697]  ? trace_hardirqs_on+0x4a/0xf0
[  217.501697]  khugepaged+0x9a9/0x20f0
[  217.501697]  ? _raw_spin_unlock+0x21/0x30
[  217.501697]  ? trace_hardirqs_on+0x4a/0xf0
[  217.501697]  ? wait_woken+0xa0/0xa0
[  217.501697]  kthread+0xf5/0x110
[  217.501697]  ? collapse_pte_mapped_thp+0x3b0/0x3b0
[  217.501697]  ? kthread_create_worker_on_cpu+0x20/0x20
[  217.501697]  ret_from_fork+0x2e/0x38

Looks like it's due to CONFIG_HIGHPTE=y pte_offset_map()->kmap_atomic() vs.
mmu_notifier_invalidate_range_start().

My naive idea would be to just reorder those things, but not sure
if there's some magic ordering constraint here. At least the machine
still boots when I do it :)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0a1b4b484ac5..f05d27b7183d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1028,12 +1028,13 @@ static void collapse_huge_page(struct mm_struct *mm,
 
 	anon_vma_lock_write(vma->anon_vma);
 
-	pte = pte_offset_map(pmd, address);
-	pte_ptl = pte_lockptr(mm, pmd);
-
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, NULL, mm,
 				address, address + HPAGE_PMD_SIZE);
 	mmu_notifier_invalidate_range_start(&range);
+
+	pte = pte_offset_map(pmd, address);
+	pte_ptl = pte_lockptr(mm, pmd);
+
 	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
 	/*
 	 * After this gup_fast can't run anymore. This also removes

-- 
Ville Syrjälä
Intel