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 08:47:57PM +0800, Andi Kleen wrote:
> On Tue, Jun 02, 2009 at 02:00:42PM +0200, Nick Piggin wrote:
> > > On Mon, Jun 01, 2009 at 01:50:46PM +0200, Nick Piggin wrote:
> > > > > Another major complexity is on calling the isolation routines to
> > > > > remove references from
> > > > >         - PTE
> > > > >         - page cache
> > > > >         - swap cache
> > > > >         - LRU list
> > > > > They more or less made some assumptions on their operating environment
> > > > > that we have to take care of.  Unfortunately these complexities are
> > > > > also not easily resolvable.
> > > > > 
> > > > > > (and few comments) of all the files in mm/. If you want to get rid
> > > > > 
> > > > > I promise I'll add more comments :)
> > > > 
> > > > OK, but they should still go in their relevant files. Or as best as
> > > > possible. Right now it's just silly to have all this here when much
> > > > of it could be moved out to filemap.c, swap_state.c, page_alloc.c, etc.
> > > 
> > > Can you be more specific what that "all this" is? 
> > 
> > The functions which take action in response to a bad page being 
> > detected. They belong with the subsystem that the page belongs
> > to. I'm amazed this is causing so much argument or confusion
> > because it is how the rest of mm/ code is arranged. OK, Hugh has
> > a point about ifdefs, but OTOH we have lots of ifdefs like this.
> 
> Well we're already calling into that subsystem, just not with
> a single function call.
> 
> > > > > > of the page and don't care what it's count or dirtyness is, then
> > > > > > truncate_inode_pages_range is the correct API to use.
> > > > > >
> > > > > > (or you could extract out some of it so you can call it directly on
> > > > > > individual locked pages, if that helps).
> > > > >  
> > > > > The patch to move over to truncate_complete_page() would like this.
> > > > > It's not a big win indeed.
> > > > 
> > > > No I don't mean to do this, but to move the truncate_inode_pages
> > > > code for truncating a single, locked, page into another function
> > > > in mm/truncate.c and then call that from here.
> > > 
> > > I took a look at that.  First there's no direct equivalent of
> > > me_pagecache_clean/dirty in truncate.c and to be honest I don't
> > > see a clean way to refactor any of the existing functions to 
> > > do the same.
> > 
> > With all that writing you could have just done it. It's really
> 
> I would have done it if it made sense to me, but so far it hasn't.
> 
> The problem with your suggestion is that you do the big picture,
> but seem to skip over a lot of details. But details matter.
> 
> > 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.
> 
> The page_mapped() check is useless because the pages are not 
> mapped here etc.
> 
> 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 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.

Yes I like the current "one code block calling one elemental function
to isolate from one reference source" scenario:
         - PTE
         - page cache
         - swap cache
         - LRU list

Calling into the generic truncate code only messes up the concepts.

Thanks,
Fengguang