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

[Petr Tesarik <ptesarik@suse.cz>]

Dne Út 18. září 2012 04:16:27 Jan Kara napsal(a):
> 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?

Same here. In fact, I believed that other filesystems only work by accident 
(because they don't have to access the mapping). I'm not even sure about the 
semantics of the swap_activate operation. Is this documented somewhere?

Petr Tesarik
SUSE Linux

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>