Re: [PATCH] [16/19] HWPOISON: Enable .remove_error_page for migration aware file systems

[Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>]

Wu Fengguang wrote:

>>However, we have a way to avoid this kind of data corruption at
>>least for ext3.  If we mount an ext3 filesystem with data=ordered
>>and data_err=abort, all I/O errors on file data block belonging to
>>the committing transaction are checked.  When I/O error is found,
>>abort journaling and remount the filesystem with read-only to
>>prevent further updates.  This kind of feature is very important
>>for mission critical systems.
> 
> Agreed. We also set PG_error, which should be enough to trigger such
> remount?

ext3 doesn't check PG_error.  Maybe we need to do:

1. trylock_buffer()
2. if step 1. succeeds, then clear_buffer_dirty(),
   clear_buffer_uptodate(), and set_buffer_write_io_error()

Note that we can't truncate the corrupted page until the error
check is done by kjournald.
 
>>If we merge this patch, we would face the data corruption problem
>>again.
>>
>>I think there are three options,
>>
>>(1) drop this patch
>>(2) merge this patch with new panic_on_dirty_page_cache_corruption
>>    sysctl
>>(3) implement a more sophisticated error_remove_page function
> 
> In fact we proposed a patch for preventing the re-corruption case, see
> 
>         http://lkml.org/lkml/2009/6/11/294
> 
> However it is hard to answer the (policy) question "How sticky should
> the EIO bit remain?".

It's a good approach!  This approach may also solve my concern,
the re-corruption issue caused by transient IO errors.

But I also think it needs a bit more consideration.  For example,
if the application has the valid data in the user space buffer,
it would try to re-write it after detecting an IO error from the
previous write.  In this case, we should clear the sticky error flag.
 
Thanks,
-- 
Hidehiro Kawai
Hitachi, Systems Development Laboratory
Linux Technology Center