linux-kernel - Re: [PATCH] udf: add extent cache support in case of file reading

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Message-ID: <20130122100447.GA31140@quack.suse.cz>
Date:	Tue, 22 Jan 2013 11:04:47 +0100
From:	Jan Kara <jack@...e.cz>
To:	Namjae Jeon <linkinjeon@...il.com>
Cc:	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Namjae Jeon <namjae.jeon@...sung.com>,
	Ashish Sangwan <a.sangwan@...sung.com>,
	Bonggil Bak <bgbak@...sung.com>
Subject: Re: [PATCH] udf: add extent cache support in case of file reading

On Tue 22-01-13 09:45:09, Namjae Jeon wrote:
> 2013/1/21, Jan Kara <jack@...e.cz>:
> > @@ -2222,6 +2219,8 @@ int udf_read_extent_cache(struct inode *inode, loff_t
> > bcount,
> >  		*lbcount = iinfo->cached_extent.lstart;
> >  		memcpy(pos, &iinfo->cached_extent.epos,
> >  		       sizeof(struct extent_position));
> > +		if (pos->bh)
> > +			get_bh(pos->bh);
> >  		spin_unlock(&iinfo->i_extent_cache_lock);
> >  		return 1;
> >  	} else
> >   This is the most important - we should give buffer reference to pos->bh.
> > Caller will eventually free it right?
> This change is not required as we give buffer reference to pos->bh at
> the time of cache update.
> When we start reading a file, first we try to read the cache which
> will lead to cache miss.
> So, we would really access the pos->bh in udf_update_extent_cache for
> the first time, and this is where the buffer reference is incremented.
> Calling get_bh at 2 places will eventually lead to mem leak.
> Let me know your opinion.
  Yes, udf_update_extent_cache() gets its own reference to bh but that is
dropped in udf_clear_extent_cache(). So I think udf_read_extent_cache()
needs to get a reference to the caller (as the caller will eventually free
the bh via brelse(epos.bh) e.g. in udf_extend_file(). Also I realized
udf_update_extent_cache() needs to first clear the cache if it is valid.
Otherwise it just overwrites bh pointer and reference is leaked. Is it
clearer now?

  I've also changed locking of udf_clear_extent_cache() so that
i_extent_cache_lock is always taken for that function - it makes the
locking rules obvious at the first sight.

  Attached is the patch I currently carry.

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

View attachment "0001-udf-add-extent-cache-support-in-case-of-file-reading.patch" of type "text/x-patch" (7356 bytes)