Re: [PATCH 03/10] ext4: fix unwritten counter leakage

On Thu, 27 Sep 2012 14:34:20 +0200, Jan Kara [off-list ref] wrote:

On Thu 27-09-12 16:19:01, Dmitry Monakhov wrote:

quoted

On Wed, 26 Sep 2012 15:07:14 +0200, Jan Kara [off-list ref] wrote:

quoted

On Mon 24-09-12 15:44:13, Dmitry Monakhov wrote:

quoted

ext4_set_io_unwritten_flag() will increment i_unwritten counter, so
once we mark end_io with END_IO_UNWRITTEN we have to revert it back

                           ^^ EXT4_IO_END_UNWRITTEN

quoted

on error path.

 - add missed error checks to prevent counter leakage
 - ext4_end_io_nolock() will clear END_IO_UNWRITTEN flag to signal

                                     ^^ EXT4_IO_END_UNWRITTEN

quoted

   that conversion finished.
 - add BUGON to free_end_io() to prevent similar leackage in future.

         ^^ BUG_ON    ^^ext4_free_io_end()         ^^ leakage

quoted

Visiable effect of this bug is that unaligned aio_stress may deadlock

  ^^ Visible

  Umm, and won't it be more foolproof it we just decrement i_unwritten in
ext4_free_io_end() when we see EXT4_IO_END_UNWRITTEN set?

I'd like to consider BUG_ON inside ext4_free_io_end as a sanity check to
force all callers to perform all necessary error checks in known context. 

  I'm not sure how "performing all necessary error checks in known context"
relates to ext4_free_io_end() cleaning up the structure on its own or
whether someone has to do it beforehand... Can you maybe elaborate a bit
more?

I assume that if end_io was tagged with UNWRITTEN flag it should goes trough
complete_io_list and end_io_nolock(), or caller should cancel it by
itself in case of error, otherwise we may miss valid unwritten end_io
but was not scheduled to complete_end_io routine by occasion (and endup
in silent data loss). In my opinion at the time then ext4_free_io_end() 
was called all possible conversions should be completed.

quoted

quoted

  That still leaves the mess with EXT4_STATE_DIO_UNWRITTEN unhandled. But
that's a separate issue. We seem to clear that flag only in
ext4_ext_direct_IO() although it could be set even when buffered write
converts extents. And error cases seem to be buggy as well.

No, each unwritten extent will be added to i_complete_io_list regardless
to it's origin (buffered or DIO), and will be completed via
ext4_end_io_nolock(). So assertion is correct.

  Yes, I agree with what you say. My note was just an off-topic rambling
about inode flag EXT4_STATE_DIO_UNWRITTEN whose handling seem to be buggy
as well.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR
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