linux-ext4 - Re: [PATCH] ext4: issue WARN_ON_ONCE() if page gets truncated in ext4_write_[da

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Message-ID: <20150517175118.GI4489@thunk.org>
Date:	Sun, 17 May 2015 13:51:18 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Xiaoguang Wang <wangxg.fnst@...fujitsu.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: issue WARN_ON_ONCE() if page gets truncated in
 ext4_write_[da_]begin()

On Thu, May 07, 2015 at 05:17:23PM +0800, Xiaoguang Wang wrote:
> In ext4_write_[da_]begin(), when we get a page successfully by calling
> grab_cache_page_write_begin(), then unless this page is truncated by
> ext4_truncate_failed_write(), which is called when ext4_write_[da_]begin()
> run into some errors, otherwise I think this page won't be truncated by
> other kernel path because we're holding i_mutex.
> 
> In this patch, if page is truncated by ext4_truncate_failed_write(), then
> we just put it and call grab_cache_page_write_begin() again to get a new
> page, then this check 'if (page->mapping != mapping)' won't be necessary.
> We can remove this check, but according to Jan Kara's suggestion, we issue
> WARN_ON_ONCE() here if page is truncated, that means something is wrong.
> (Thanks to Jan Kara for his help)

It would be helpful if the commit description explained why this
change is a benefit.  There's a very detailed explanation about why
it's safe, but why do we need to release the page only to try again to
grab it by jumping to retry_grab instead of retry_journal?  What
problem are you trying to solve?

Thanks, regards,

					- Ted
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