inactive buffer head in lrus prevents page migrate

[lijiazi <jqqlijiazi@xxxxxxxxx>]

hi Al Viro,

I recently encountered a CMA page migration failure issue.
This page has private, and private data is buffer_head struct pointer.
buffer_head->b_count is not zero, so drop_buffers failed.

This leads to the failure of both directly reclaim and migration attempts for
this page.
Finally, CMA memory alloc failed.

This buffer_head detail info are as follows:

crash> struct buffer_head 0xffffffec9f0200d0 -x
struct buffer_head {
  b_state = 0x29, //has Uptodate, Req, Mapped flags
  b_this_page = 0xffffffec9f0200d0,
  b_page = 0xffffffbfb4bb0080,
  b_blocknr = 0x801b,
  b_size = 0x1000,
  b_data = 0xffffffed2ec02000 "\244\201",
  b_bdev = 0xffffffed169b2580,
  b_end_io = 0xffffff91006c44e4 <end_buffer_read_sync>,
  b_private = 0x0,
  b_assoc_buffers = {
    next = 0xffffffec9f020118,
    prev = 0xffffffec9f020118
  },
  b_assoc_map = 0x0,
  b_count = {
    counter = 0x1
  }
}

The b_count is 1, just because it's in cpu6 bh_lru:
crash> p bh_lrus:a | grep 0xffffffec9f0200d0 -B 1
per_cpu(bh_lrus, 6) = $7 = {
  bhs = {0xffffffed146867b8, 0xffffffec9f020548, 0xffffffed0f7e3138, 0xffffffed0f7e30d0,
	 0xffffffed0f6f7340, 0xffffffed0b8c59c0, 0xffffffeb7bdb7888, 0xffffffed0b8c5548,
	 0xffffffed0f7b7270, 0xffffffed0f7b7208, 0xffffffed0f7b7138, 0xffffffec9f0200d0,//this entry
	 0xffffffed0f7b7068, 0xffffffed0f7b7000, 0xffffffed0f7b7bc8, 0xffffffec9f020068}

On my device using kernel-4.19, inactive bh may be in bh_lrus for a long
time, and cause the corresponding page migration failure.

In function buffer_busy, can we check if b_count is greater than zero
just because it's in bh_lrus?
If yes, can we evict some inactive bhs to improve the success rate of
migration?

Thank you very much!