UBIFS: Failed to read/write file when contiguous pages are not enough

[yhb at ruijie.com.cn]

There is only one memory zone:
DMA32 free:88712kB min:45568kB low:56960kB high:68352kB active_anon:490248kB inactive_anon:192712kB active_file:8852kB inactive_file:21268kB unevictable:0kB isolated(anon):56kB isolated(file):3272kB present:2028660kB mlocked:0kB dirty:24512kB writeback:20kB mapped:16960kB shmem:17936kB slab_reclaimable:5160kB slab_unreclaimable:1147384kB kernel_stack:12736kB pagetables:8428kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:52723 all_unreclaimable? yes
lowmem_reserve[]: 0 0 0
DMA32: 19314*4kB 982*8kB 191*16kB 15*32kB 1*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB 0*8192kB 0*16384kB 0*32768kB = 88712kB
The number of free pages is above high watermark, but the number of the free pages with order >= 1 is below half of min watermark.

UBIFS need allocate two contiguous pages when it reads file.
do_readpage():
dn = kmalloc(UBIFS_MAX_DATA_NODE_SZ, GFP_NOFS);

UBIFS need allocate four contiguous pages when it writes file.
do_writepage -> ubifs_jnl_write_data:
data = kmalloc(dlen, GFP_NOFS | __GFP_NOWARN);

When the free pages with order >= 1 are not enough, many processes hang in the page fault handler.
 Call Trace:
[<a80000008a947560 : ffffffff8010d4c8>] schedule+0x348/0x980
[<a80000008a947620 : ffffffff8010de9c>] schedule_timeout+0x174/0x228
[<a80000008a9476a0 : ffffffff8010cd54>] io_schedule_timeout+0x6c/0x108
[<a80000008a9476c0 : ffffffff8035394c>] congestion_wait+0x7c/0xa8
[<a80000008a947710 : ffffffff8034bf64>] shrink_zone+0xbac/0xea8
[<a80000008a9478f0 : ffffffff8034ccfc>] do_try_to_free_pages+0xf4/0x428
[<a80000008a947960 : ffffffff8034d128>] try_to_free_pages+0x68/0x78
[<a80000008a9479c0 : ffffffff80344730>] __alloc_pages_nodemask+0x358/0x5f8
[<a80000008a947aa0 : ffffffff8036faf4>] cache_alloc_refill+0x3ac/0x730
[<a80000008a947b10 : ffffffff80370098>] kmem_cache_alloc+0xd0/0xd8
[<a80000008a947b40 : ffffffff80410c54>] do_readpage+0x15c/0x3d8
[<a80000008a947ba0 : ffffffff804118a4>] ubifs_readpage+0x5c/0x3f0
[<a80000008a947c40 : ffffffff8033df2c>] filemap_fault+0x294/0x410
[<a80000008a947c90 : ffffffff803568c0>] __do_fault+0x70/0x738
[<a80000008a947d30 : ffffffff803596b8>] handle_mm_fault+0x1e0/0xf80
[<a80000008a947dc0 : ffffffff802dee5c>] do_page_fault+0x13c/0x388
[<a80000008a947e80 : ffffffff80100980>] ret_from_exception+0x0/0x8

UBIFS needs improvement: do not allocate contiguous pages.