powerpc/mm: Add cond_resched() while removing hpte mappings

While removing large number of mappings from hash page tables for
large memory systems as soft-lockup is reported because of the time
spent inside htap_remove_mapping() like one below:

 watchdog: BUG: soft lockup - CPU#8 stuck for 23s!
 <snip>
 NIP plpar_hcall+0x38/0x58
 LR  pSeries_lpar_hpte_invalidate+0x68/0xb0
 Call Trace:
  0x1fffffffffff000 (unreliable)
  pSeries_lpar_hpte_removebolted+0x9c/0x230
  hash__remove_section_mapping+0xec/0x1c0
  remove_section_mapping+0x28/0x3c
  arch_remove_memory+0xfc/0x150
  devm_memremap_pages_release+0x180/0x2f0
  devm_action_release+0x30/0x50
  release_nodes+0x28c/0x300
  device_release_driver_internal+0x16c/0x280
  unbind_store+0x124/0x170
  drv_attr_store+0x44/0x60
  sysfs_kf_write+0x64/0x90
  kernfs_fop_write+0x1b0/0x290
  __vfs_write+0x3c/0x70
  vfs_write+0xd4/0x270
  ksys_write+0xdc/0x130
  system_call+0x5c/0x70

Fix this by adding a cond_resched() to the loop in
htap_remove_mapping() that issues hcall to remove hpte mapping. The
call to cond_resched() is issued every HZ jiffies which should prevent
the soft-lockup from being reported.
Suggested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210404163148.321346-1-vaibhav@linux.ibm.com

arch/powerpc/mm/book3s64/hash_utils.c

@@ -338,7 +338,7 @@ int htab_bolt_mapping(unsigned long vstart, unsigned long vend,
 int htab_remove_mapping(unsigned long vstart, unsigned long vend,
 		      int psize, int ssize)
 {
-	unsigned long vaddr;
+	unsigned long vaddr, time_limit;
 	unsigned int step, shift;
 	int rc;
 	int ret = 0;
@@ -351,8 +351,19 @@ int htab_remove_mapping(unsigned long vstart, unsigned long vend,
 
 	/* Unmap the full range specificied */
 	vaddr = ALIGN_DOWN(vstart, step);
+	time_limit = jiffies + HZ;
+
 	for (;vaddr < vend; vaddr += step) {
 		rc = mmu_hash_ops.hpte_removebolted(vaddr, psize, ssize);
+
+		/*
+		 * For large number of mappings introduce a cond_resched()
+		 * to prevent softlockup warnings.
+		 */
+		if (time_after(jiffies, time_limit)) {
+			cond_resched();
+			time_limit = jiffies + HZ;
+		}
 		if (rc == -ENOENT) {
 			ret = -ENOENT;
 			continue;