mm: use memalloc_nofs_save() in page_cache_ra_order()

See commit f2c817b ("mm: use memalloc_nofs_save in readahead path"),
ensure that page_cache_ra_order() do not attempt to reclaim file-backed
pages too, or it leads to a deadlock, found issue when test ext4 large
folio.

 INFO: task DataXceiver for:7494 blocked for more than 120 seconds.
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 task:DataXceiver for state:D stack:0     pid:7494  ppid:1      flags:0x00000200
 Call trace:
  __switch_to+0x14c/0x240
  __schedule+0x82c/0xdd0
  schedule+0x58/0xf0
  io_schedule+0x24/0xa0
  __folio_lock+0x130/0x300
  migrate_pages_batch+0x378/0x918
  migrate_pages+0x350/0x700
  compact_zone+0x63c/0xb38
  compact_zone_order+0xc0/0x118
  try_to_compact_pages+0xb0/0x280
  __alloc_pages_direct_compact+0x98/0x248
  __alloc_pages+0x510/0x1110
  alloc_pages+0x9c/0x130
  folio_alloc+0x20/0x78
  filemap_alloc_folio+0x8c/0x1b0
  page_cache_ra_order+0x174/0x308
  ondemand_readahead+0x1c8/0x2b8
  page_cache_async_ra+0x68/0xb8
  filemap_readahead.isra.0+0x64/0xa8
  filemap_get_pages+0x3fc/0x5b0
  filemap_splice_read+0xf4/0x280
  ext4_file_splice_read+0x2c/0x48 [ext4]
  vfs_splice_read.part.0+0xa8/0x118
  splice_direct_to_actor+0xbc/0x288
  do_splice_direct+0x9c/0x108
  do_sendfile+0x328/0x468
  __arm64_sys_sendfile64+0x8c/0x148
  invoke_syscall+0x4c/0x118
  el0_svc_common.constprop.0+0xc8/0xf0
  do_el0_svc+0x24/0x38
  el0_svc+0x4c/0x1f8
  el0t_64_sync_handler+0xc0/0xc8
  el0t_64_sync+0x188/0x190

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 793917d ("mm/readahead: Add large folio readahead")
Signed-off-by: Kefeng Wang <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Zhang Yi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Kefeng Wang

mm/readahead.c

@@ -490,6 +490,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	pgoff_t index = readahead_index(ractl);
 	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
 	pgoff_t mark = index + ra->size - ra->async_size;
+	unsigned int nofs;
 	int err = 0;
 	gfp_t gfp = readahead_gfp_mask(mapping);
 
@@ -504,6 +505,8 @@ void page_cache_ra_order(struct readahead_control *ractl,
 		new_order = min_t(unsigned int, new_order, ilog2(ra->size));
 	}
 
+	/* See comment in page_cache_ra_unbounded() */
+	nofs = memalloc_nofs_save();
 	filemap_invalidate_lock_shared(mapping);
 	while (index <= limit) {
 		unsigned int order = new_order;
@@ -527,6 +530,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 
 	read_pages(ractl);
 	filemap_invalidate_unlock_shared(mapping);
+	memalloc_nofs_restore(nofs);
 
 	/*
 	 * If there were already pages in the page cache, then we may have