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Date:   Thu, 11 Jun 2020 10:21:03 +0200
From:   Jan Kara <jack@...e.cz>
To:     "zhangyi (F)" <yi.zhang@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, "Theodore Y. Ts'o" <tytso@....edu>,
        linux-ext4@...r.kernel.org, adilger.kernel@...ger.ca,
        zhangxiaoxu5@...wei.com
Subject: Re: [PATCH 00/10] ext4: fix inconsistency since reading old metadata
 from disk

On Thu 11-06-20 10:12:45, zhangyi (F) wrote:
> On 2020/6/11 0:27, Jan Kara wrote:
> > On Wed 10-06-20 11:45:43, Theodore Y. Ts'o wrote:
> >> On Wed, Jun 10, 2020 at 11:57:39AM +0200, Jan Kara wrote:
> >>>> So I guess it may still lead to inconsistency. How about add this checking
> >>>> into ext4_journal_get_write_access() ?
> >>>
> >>> Yes, this also occured to me later. Adding the check to
> >>> ext4_journal_get_write_access() should be safer.
> >>
> >> There's another thing which we could do.  One of the issues is that we
> >> allow buffered writeback for block devices once the change to the
> >> block has been committed.  What if we add a change to block device
> >> writeback code and in fs/buffer.c so that optionally, the file system
> >> can specify a callback function can get called when an I/O error has
> >> been reflected back up from the block layer?
> >>
> >> It seems unfortunate that currently, we can immediately report the I/O
> >> error for buffered writes to *files*, but for metadata blocks, we
> >> would only be able to report the problem when we next try to modify
> >> it.
> >>
> >> Making changes to fs/buffer.c might be controversial, but I think it
> >> might be result in a better solution.
> > 
> > Yeah, what you propose certainly makes sence could be relatively easily
> > done by blkdev_writepage() using __block_write_full_page() with appropriate
> > endio handler which calls fs callback. I'm just not sure how propagate the
> > callback function from the fs to the blkdev...
> >
> 
> I have thought about this solution, we could add a hook in 'struct super_operations'
> and call it in blkdev_writepage() like blkdev_releasepage() does, and pick out a
> wrapper from block_write_full_page() to pass our endio handler in, something like
> this.
> 
> static const struct super_operations ext4_sops = {
> ...
> 	.bdev_write_page = ext4_bdev_write_page,
> ...
> };
> 
> static int blkdev_writepage(struct page *page, struct writeback_control *wbc)
> {
> 	struct super_block *super = BDEV_I(page->mapping->host)->bdev.bd_super;
> 
> 	if (super && super->s_op->bdev_write_page)
> 		return super->s_op->bdev_write_page(page, blkdev_get_block, wbc);
> 
> 	return block_write_full_page(page, blkdev_get_block, wbc);
> }
> 
> But I'm not sure it's a optimal ieda. So I continue to realize the "wb_err"
> solution now ?

The above idea looks good to me. I'm fine with either that solution or
"wb_err" idea so maybe let's leave it for Ted to decide...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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