[Bug 201461] ext4 journal stalls write system call

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=201461

--- Comment #3 from Theodore Tso (tytso@mit.edu) ---
The writeback code will end up calling ext4_writepages(), which will do block
allocations via ext4_map_blocks() with the flag EXT4_GET_BLOCKS_CREATE(), and
this will take a write lock on i_data_sem.   

I'll let you trace through the call chain that starts in the writeback thread,
through the call to ext4_writepages(), proceeds to mpage_map_one_extent(), and
from there to ext4_map_blocks() which actually holds the write block.

Note that this is distinct from the data=ordered forced writeblock which starts
in fs/jbd2's journal_submit_inode_data_buffer() calling generic_writepages().  
The difference is that generic_writepages() will only call writepage()
function.  For ext4 that's ext4_writepage(), which will never do block
allocation.

But the writeback threads call the address space operation's, writepages()
function, and ext4_writepages() *will* do block allocation for delayed
allocation writes.

This is a subtle point; ext4_writepage() and ext4_writepages() do very
different things.   The first is used by the commit kernel thread; the second
is used by the writeback kernel thread.

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