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Date:   Mon, 8 Jun 2020 22:39:31 +0800
From:   "zhangyi (F)" <yi.zhang@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <linux-ext4@...r.kernel.org>, <tytso@....edu>,
        <adilger.kernel@...ger.ca>, <zhangxiaoxu5@...wei.com>
Subject: Re: [PATCH 00/10] ext4: fix inconsistency since reading old metadata
 from disk

Hi, Jan.

On 2020/6/8 16:20, Jan Kara wrote:
> Hello Yi!
> 
> On Tue 26-05-20 15:17:44, zhangyi (F) wrote:
>> Background
>> ==========
>>
>> This patch set point to fix the inconsistency problem which has been
>> discussed and partial fixed in [1].
>>
>> Now, the problem is on the unstable storage which has a flaky transport
>> (e.g. iSCSI transport may disconnect few seconds and reconnect due to
>> the bad network environment), if we failed to async write metadata in
>> background, the end write routine in block layer will clear the buffer's
>> uptodate flag, but the data in such buffer is actually uptodate. Finally
>> we may read "old && inconsistent" metadata from the disk when we get the
>> buffer later because not only the uptodate flag was cleared but also we
>> do not check the write io error flag, or even worse the buffer has been
>> freed due to memory presure.
>>
>> Fortunately, if the jbd2 do checkpoint after async IO error happens,
>> the checkpoint routine will check the write_io_error flag and abort the
>> the journal if detect IO error. And in the journal recover case, the
>> recover code will invoke sync_blockdev() after recover complete, it will
>> also detect IO error and refuse to mount the filesystem.
>>
>> Current ext4 have already deal with this problem in __ext4_get_inode_loc()
>> and commit 7963e5ac90125 ("ext4: treat buffers with write errors as
>> containing valid data"), but it's not enough.
> 
> Before we go and complicate ext4 code like this, I'd like to understand
> what is the desired outcome which doesn't seem to be mentioned here, in the
> commit 7963e5ac90125, or in the discussion you reference. If you have a
> flaky transport that gives you IO errors, IMO it is not a bussiness of the
> filesystem to try to fix that. I just has to make sure it properly reports

If we meet some IO errors due to the flaky transport, IMO the desired outcome
is 1) report IO error; 2) ext4 filesystem act as the "errors=xxx" configuration
specified, if we set "errors=read-only || panic", we expect ext4 could remount
to read-only or panic immediately to avoid inconsistency. In brief, the kernel
should try best to guarantee the filesystem on disk is consistency, this will
reduce fsck's work (AFAIK, the fsck cannot fix those inconsistent in auto mode
for most cases caused by the async error problem I mentioned), so we could
recover the fs automatically in next boot.

But now, in the case of metadata async writeback, (1) is done in
end_buffer_async_write(), but (2) is not guaranteed, because ext4 cannot detect
metadata write error, and it also cannot remount the filesystem or invoke panic
immediately. Finally, if we read the metadata on disk and re-write again, it
may lead to on-disk filesystem inconsistency.

> errors to userspace and (depending of errors= configuration) shuts itself
> down to limit further damage. This patch seems to try to mask those errors
> and that's, in my opinion, rather futile (as in you can hardly ever deal
> with all the cases). BTW are you running these systems on flaky iSCSI with
> errors=continue so that the errors don't shut the filesystem down
> immediately?
> 
Yes, I run ext4 filesystem on a flaky iSCSI(it is stable most of the time)
with errors=read-only, in the cases mentioned above, the fs will not be
remount to read-only immediately or remount after it has already been
inconsistency.

Thinking about how to fix this, one method is to invoke ext4_error() or
jbd2_journal_abort() when we detect write error to prevent further use of
the filesystem. But after looking at __ext4_get_inode_loc() and 7963e5ac90125,
I think although the metadata buffer was failed to write back to the disk due
to the occasional unstable network environment, but the data in the buffer
is actually uptodate, the filesystem could self-healing after the network
recovery. In the worst case, if the network is broken for a long time, the
jbd2's checkpoint routing will detect the error, or jbd2 will failed to write
the journal to disk, both will abort the filesystem. So I think we could
re-set the uptodate flag when we read buffer again as 7963e5ac90125 does.

Thanks,
Yi.

> 
>> [1] https://lore.kernel.org/linux-ext4/20190823030207.GC8130@mit.edu/
>>
>> Description
>> ===========
>>
>> This patch set add and rework 7 wrapper functions of getting metadata
>> blocks, replace all sb_bread() / sb_getblk*() / ext4_bread() and
>> sb_breadahead*(). Add buffer_write_io_error() checking into them, if
>> the buffer isn't uptodate and write_io_error flag was set, which means
>> that the buffer has been failed to write out to disk, re-add the
>> uptodate flag to prevent subsequent read operation.
>>
>>  - ext4_sb_getblk(): works the same as sb_getblk(), use to replace all
>>    sb_getblk() used for newly allocated blocks and getting buffers.
>>  - ext4_sb_getblk_locked(): works the same as sb_getblk() except check &
>>    fix buffer uotpdate flag, use to replace all sb_getblk() used for
>>    getting buffers to read.
>>  - ext4_sb_getblk_gfp(): gfp version of ext4_sb_getblk().
>>  - ext4_sb_getblk_locked_gfp(): gfp version of ext4_sb_getblk_locked().
>>  - ext4_sb_bread(): get buffer and submit read bio if buffer is actually
>>    not uptodate.
>>  - ext4_sb_bread_unmovable(): unmovable version of ext4_sb_bread().
>>  - ext4_sb_breadahead_unmovable(): works the same to ext4_sb_bread_unmovable()
>>    except skip submit read bio if failed to lock the buffer.
>>
>> Patch 1-2: do some small change in ext4 inode eio simulation and add a
>> helper in buffer.c, just prepare for below patches.
>> Patch 3: add the ext4_sb_*() function to deal with the write_io_error
>> flag in buffer.
>> Patch 4-8: replace all sb_*() with ext4_sb_*() in ext4.
>> Patch 9: deal with the buffer shrinking case, abort jbd2/fs when
>> shrinking a buffer with write_io_error flag.
>> Patch 10: just do some cleanup.
>>
>> After this patch set, we need to use above 7 wrapper functions to
>> get/read metadata block instead of invoke sb_*() functions defined in
>> fs/buffer.h.
>>
>> Test
>> ====
>>
>> This patch set is based on linux-5.7-rc7 and has been tests by xfstests
>> in auto mode.
>>
>> Thanks,
>> Yi.
>>
>>
>> zhangyi (F) (10):
>>   ext4: move inode eio simulation behind io completeion
>>   fs: pick out ll_rw_one_block() helper function
>>   ext4: add ext4_sb_getblk*() wrapper functions
>>   ext4: replace sb_getblk() with ext4_sb_getblk_locked()
>>   ext4: replace sb_bread*() with ext4_sb_bread*()
>>   ext4: replace sb_getblk() with ext4_sb_getblk()
>>   ext4: switch to use ext4_sb_getblk_locked() in ext4_getblk()
>>   ext4: replace sb_breadahead() with ext4_sb_breadahead()
>>   ext4: abort the filesystem while freeing the write error io buffer
>>   ext4: remove unused parameter in jbd2_journal_try_to_free_buffers()
>>
>>  fs/buffer.c                 |  41 ++++++----
>>  fs/ext4/balloc.c            |   6 +-
>>  fs/ext4/ext4.h              |  60 ++++++++++++---
>>  fs/ext4/extents.c           |  13 ++--
>>  fs/ext4/ialloc.c            |   6 +-
>>  fs/ext4/indirect.c          |  13 ++--
>>  fs/ext4/inline.c            |   2 +-
>>  fs/ext4/inode.c             |  53 +++++--------
>>  fs/ext4/mmp.c               |   2 +-
>>  fs/ext4/resize.c            |  24 +++---
>>  fs/ext4/super.c             | 145 +++++++++++++++++++++++++++++++-----
>>  fs/ext4/xattr.c             |   4 +-
>>  fs/jbd2/transaction.c       |  20 +++--
>>  include/linux/buffer_head.h |   1 +
>>  include/linux/jbd2.h        |   3 +-
>>  15 files changed, 277 insertions(+), 116 deletions(-)
>>
>> -- 
>> 2.21.3
>>

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