Re: Does swap_set_page_dirty() calling ->set_page_dirty() make sense?

[Jan Kara <jack@suse.cz>]

On Mon 17-09-12 12:15:46, Hugh Dickins wrote:
> On Mon, 17 Sep 2012, Jan Kara wrote:
> > 
> >   I tripped over a crash in reiserfs which happened due to PageSwapCache
> > page being passed to reiserfs_set_page_dirty(). Now it's not that hard to
> > make reiserfs_set_page_dirty() check that case but I really wonder: Does it
> > make sense to call mapping->a_ops->set_page_dirty() for a PageSwapCache
> > page? The page is going to be written via direct IO so from the POV of the
> > filesystem there's no need for any dirtiness tracking. Also there are
> > several ->set_page_dirty() implementations which will spectacularly crash
> > because they do things like page->mapping->host, or call
> > __set_page_dirty_buffers() which expects buffer heads in page->private.
> > Or what is the reason for calling filesystem's set_page_dirty() function?
> 
> This is a question for Mel, really: it used not to call the filesystem.
> 
> But my reading of the 3.6 code says that it still will not call the
> filesystem, unless the filesystem (only nfs) provides a swap_activate
> method, which should be the only case in which SWP_FILE gets set.
> And I rather think Mel does want to use the filesystem set_page_dirty
> in that case.  Am I misreading?
> 
> Did you see this on a vanilla kernel?  Or is it possible that you have
> a private patch merged in, with something else sharing the SWP_FILE bit
> (defined in include/linux/swap.h) by mistake?
  Argh, sorry. It is indeed a SLES specific bug. I missed that SWP_FILE bit
gets set only when swap_activate() is provided (SLES code works a bit
differently in this area but I wasn't really looking into that since I was
focused elsewhere).

So just one minor nit for Mel. SWP_FILE looks like a bit confusing name for
a flag that gets set only for some swap files ;) At least I didn't pay
attention to it because I thought it's set for all of them. Maybe call it
SWP_FILE_CALL_AOPS or something like that?

Thanks Hugh for having a look.

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

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