[PATCH RFC] Ext4: fix deadlock on dirty pages between fault and writeback

[Liu Bo <bo.liu@linux.alibaba.com>]

mpage_prepare_extent_to_map() tries to build up a large bio to stuff down
the pipe.  But if it needs to wait for a page lock, it needs to make sure
and send down any pending writes so we don't deadlock with anyone who has
the page lock and is waiting for writeback of things inside the bio.

The related lock stack is shown as follows,

task1:
[<ffffffff811aaa52>] wait_on_page_bit+0x82/0xa0
[<ffffffff811c5777>] shrink_page_list+0x907/0x960
[<ffffffff811c6027>] shrink_inactive_list+0x2c7/0x680
[<ffffffff811c6ba4>] shrink_node_memcg+0x404/0x830
[<ffffffff811c70a8>] shrink_node+0xd8/0x300
[<ffffffff811c73dd>] do_try_to_free_pages+0x10d/0x330
[<ffffffff811c7865>] try_to_free_mem_cgroup_pages+0xd5/0x1b0
[<ffffffff8122df2d>] try_charge+0x14d/0x720
[<ffffffff812320cc>] memcg_kmem_charge_memcg+0x3c/0xa0
[<ffffffff812321ae>] memcg_kmem_charge+0x7e/0xd0
[<ffffffff811b68a8>] __alloc_pages_nodemask+0x178/0x260
[<ffffffff8120bff5>] alloc_pages_current+0x95/0x140
[<ffffffff81074247>] pte_alloc_one+0x17/0x40
[<ffffffff811e34de>] __pte_alloc+0x1e/0x110
[<ffffffffa06739de>] alloc_set_pte+0x5fe/0xc20
[<ffffffff811e5d93>] do_fault+0x103/0x970
[<ffffffff811e6e5e>] handle_mm_fault+0x61e/0xd10
[<ffffffff8106ea02>] __do_page_fault+0x252/0x4d0
[<ffffffff8106ecb0>] do_page_fault+0x30/0x80
[<ffffffff8171bce8>] page_fault+0x28/0x30
[<ffffffffffffffff>] 0xffffffffffffffff

task2:
[<ffffffff811aadc6>] __lock_page+0x86/0xa0
[<ffffffffa02f1e47>] mpage_prepare_extent_to_map+0x2e7/0x310 [ext4]
[<ffffffffa08a2689>] ext4_writepages+0x479/0xd60
[<ffffffff811bbede>] do_writepages+0x1e/0x30
[<ffffffff812725e5>] __writeback_single_inode+0x45/0x320
[<ffffffff81272de2>] writeback_sb_inodes+0x272/0x600
[<ffffffff81273202>] __writeback_inodes_wb+0x92/0xc0
[<ffffffff81273568>] wb_writeback+0x268/0x300
[<ffffffff81273d24>] wb_workfn+0xb4/0x390
[<ffffffff810a2f19>] process_one_work+0x189/0x420
[<ffffffff810a31fe>] worker_thread+0x4e/0x4b0
[<ffffffff810a9786>] kthread+0xe6/0x100
[<ffffffff8171a9a1>] ret_from_fork+0x41/0x50
[<ffffffffffffffff>] 0xffffffffffffffff

task1 is waiting for the PageWriteback bit of the page that task2 has
collected in mpd->io_submit->io_bio, and tasks2 is waiting for the LOCKED
bit the page which tasks1 has locked.

It seems that this deadlock only happens when those pages are mapped pages
so that mpage_prepare_extent_to_map() can have pages queued in io_bio and
when waiting to lock the subsequent page.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
---

Only did build test.

 fs/ext4/inode.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index c3d9a42c561e..becbfb292bf0 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2681,7 +2681,26 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
 			if (mpd->map.m_len > 0 && mpd->next_page != page->index)
 				goto out;
 
-			lock_page(page);
+			if (!trylock_page(page)) {
+				/*
+				 * A rare race may happen between fault and
+				 * writeback,
+				 *
+				 * 1. fault may have raced in and locked this
+				 * page ahead of us, and if fault needs to
+				 * reclaim memory via shrink_page_list(), it may
+				 * also wait on the writeback pages we've
+				 * collected in our mpd->io_submit.
+				 *
+				 * 2. We have to submit mpd->io_submit->io_bio
+				 * to let memory reclaim make progress in order
+				 * to avoid the deadlock between fault and
+				 * ourselves(writeback).
+				 */
+				ext4_io_submit(&mpd->io_submit);
+				lock_page(page);
+			}
+
 			/*
 			 * If the page is no longer dirty, or its mapping no
 			 * longer corresponds to inode we are writing (which
-- 
1.8.3.1