Re: EXT3: problem with copy_from_user inside a transaction

[Andrey Savochkin <saw@saw.sw.com.sg>]

On Sat, Sep 04, 2004 at 03:47:44AM -0400, Chris Mason wrote:
> On Fri, 2004-09-03 at 09:57, Andrey Savochkin wrote:
> 
> > > This would mean that all the work is done during the commit_write
> > > stage.  The trick is that we would have to handle -ENOSPC since we might
> > > not know we've run out of room until after the data has been copied from
> > > userland.
> > 
> > What is the problem -ENOSPC?
> > Do you think about the problem of the page existing before this write, it's
> > content overwritten, but the filesystem being unable to commit that write
> > because it needs more space?
> 
> Exactly.  In this case, we've effectively corrupted the page cache. 
> We've copied data in that isn't (and never will be) reflected on disk. 
> It isn't a horribly difficult case, we just need to overwrite the data
> with zeros, making sure to only overwrite the data corresponding to the
> -ENOSPC error.

Zeroing not mapped buffers in case of error is not difficult, indeed.

I was speaking about the following case:
 - one page of a file is dirtied through a userspace mapping,
 - the page doesn't have disk mapping yet (a hole),
 - someone issues write() to this page,
 - the space allocation fails in commit_write(), when the page content has
   already been overwritten with the new data.

Any holes in this scenario? :)

How to handle -ENOSPC in this case?

	Andrey