memremap: add scheduling point to devm_memremap_pages

devm_memremap_pages is initializing struct pages in for_each_device_pfn
and that can take quite some time.  We have even seen a soft lockup
triggering on a non preemptive kernel

  NMI watchdog: BUG: soft lockup - CPU#61 stuck for 22s! [kworker/u641:11:1808]
  [...]
  RIP: 0010:[<ffffffff8118b6b7>]  [<ffffffff8118b6b7>] devm_memremap_pages+0x327/0x430
  [...]
  Call Trace:
    pmem_attach_disk+0x2fd/0x3f0 [nd_pmem]
    nvdimm_bus_probe+0x64/0x110 [libnvdimm]
    driver_probe_device+0x1f7/0x420
    bus_for_each_drv+0x52/0x80
    __device_attach+0xb0/0x130
    bus_probe_device+0x87/0xa0
    device_add+0x3fc/0x5f0
    nd_async_device_register+0xe/0x40 [libnvdimm]
    async_run_entry_fn+0x43/0x150
    process_one_work+0x14e/0x410
    worker_thread+0x116/0x490
    kthread+0xc7/0xe0
    ret_from_fork+0x3f/0x70

fix this by adding cond_resched every 1024 pages.

Link: http://lkml.kernel.org/r/20170918121410.24466-4-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Johannes Thumshirn <jthumshirn@suse.de>
Tested-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Dan Williams <dan.j.williams@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/memremap.c

@@ -350,7 +350,7 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
 	pgprot_t pgprot = PAGE_KERNEL;
 	struct dev_pagemap *pgmap;
 	struct page_map *page_map;
-	int error, nid, is_ram;
+	int error, nid, is_ram, i = 0;
 
 	align_start = res->start & ~(SECTION_SIZE - 1);
 	align_size = ALIGN(res->start + resource_size(res), SECTION_SIZE)
@@ -448,6 +448,8 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
 		list_del(&page->lru);
 		page->pgmap = pgmap;
 		percpu_ref_get(ref);
+		if (!(++i % 1024))
+			cond_resched();
 	}
 	devres_add(dev, page_map);
 	return __va(res->start);