[Bug 201461] ext4 journal stalls write system call

[bugzilla-daemon@bugzilla.kernel.org]

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

--- Comment #4 from zhenbianshu@foxmail.com ---
(In reply to Theodore Tso from comment #3)
> 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.

I did not find ext4_writepages() or mpage_map_one_extent() in linux 3.10.0
source code, what I found is ext4_da_writepages().

And the call chain:
journal_submit_data_buffers()
journal_submit_inode_data_buffers()
generic_writepages()
ext4_da_writepages()
write_cache_page_da()
mpage_da_map_and_submit()
ext4_map_blocks()
will take the write lock of i_data_sem.

Will generic_writepages() call ext4_da_writepages() in jbd2 ?

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