Re: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v3

[Wu Fengguang <fengguang.wu@intel.com>]

On Tue, Jun 02, 2009 at 08:57:13PM +0800, Nick Piggin wrote:
> On Tue, Jun 02, 2009 at 02:47:57PM +0200, Andi Kleen wrote:
> > On Tue, Jun 02, 2009 at 02:00:42PM +0200, Nick Piggin wrote:
> > > not a big deal and just avoids duplicating code. I attached an
> > > (untested) patch.
> > 
> > Thanks. But the function in the patch is not doing the same what
> > the me_pagecache_clean/dirty are doing. For once there is no error
> > checking, as in the second try_to_release_page()
> > 
> > Then it doesn't do all the IO error and missing mapping handling.
> 
> Obviously I don't mean just use that single call for the entire
> handler. You can set the EIO bit or whatever you like. The
> "error handling" you have there also seems strange. You could
> retain it, but the page is assured to be removed from pagecache.

You mean this?

        if (page_has_private(p) && !try_to_release_page(p, GFP_NOIO))
                return FAILED;

If page->private cannot be removed, that means some fs may start IO on it, so
we return FAILED.

> > The page_mapped() check is useless because the pages are not 
> > mapped here etc.
> 
> That's OK, it is a core part of the protocol to prevent
> truncated pages from being mapped, so I like it to be in
> that function.
 
Right.

> (you are also doing extraneous page_mapped tests in your handler,
> so surely your concern isn't from the perspective of this
> error handler code)
 
That's because the initial try_to_unmap() may fail and page still remain
mapped, and remove_from_page_cache() assumes !page_mapped(). 

> > We could probably call truncate_complete_page(), but then
> > we would also need to duplicate most of the checking outside
> > the function anyways and there wouldn't be any possibility
> > to share the clean/dirty variants. If you insist I can
> > do it, but I think it would be significantly worse code
> > than before and I'm reluctant to do that.
> 
> I can write you the patch for that too if you like.

I have already posted one on truncate_complete_page(). Not the way you want it?
 
> > I don't also really see what the big deal is of just
> > calling these few functions directly. After all we're not
> > truncating here and they're all already called from other files.
> >
> > > > > No, it seems rather insane to do something like this here that no other
> > > > > code in the mm ever does.
> > > > 
> > > > Just because the rest of the VM doesn't do it doesn't mean it might make sense.
> > > 
> > > It is going to be possible to do it somehow surely, but it is insane
> > > to try to add such constraints to the VM to close a few small windows
> > 
> > We don't know currently if they are small. If they are small I would
> > agree with you, but that needs numbers. That said fancy writeback handling
> > is currently not on my agenda.
> 
> Yes, writeback pages are very limited, a tiny number at any time and
> the faction gets relatively smaller as total RAM size gets larger.

Yes they are less interesting for now.
 
> > > if you already have other large ones.
> > 
> > That's unclear too.
> 
> You can't do much about most kernel pages, and dirty metadata pages
> are both going to cause big problems. User pagetable pages. Lots of
> stuff.

Yes, that's a network of pointers that's hard to break away with.

Thanks,
Fengguang

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