Re: [PATCH 5/8] vmalloc: simplify vread()/vwrite()

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

On Thu, Jan 14, 2010 at 05:45:26AM -0700, Nick Piggin wrote:
> On Wed, Jan 13, 2010 at 09:53:10PM +0800, Wu Fengguang wrote:
> > vread()/vwrite() is only called from kcore/kmem to access one page at a time.
> > So the logic can be vastly simplified.
> > 
> > The changes are:
> > - remove the vmlist walk and rely solely on vmalloc_to_page()
> > - replace the VM_IOREMAP check with (page && page_is_ram(pfn))
> > - rename to vread_page()/vwrite_page()
> > 
> > The page_is_ram() check is necessary because kmap_atomic() is not
> > designed to work with non-RAM pages.
> 
> I don't know if you can really do this. Previously vmlist_lock would be
> taken, which will prevent these vm areas from being freed.
>  
> > Note that even for a RAM page, we don't own the page, and cannot assume
> > it's a _PAGE_CACHE_WB page.
> 
> So why is this not a problem for your patch? I don't see how you handle
> it.

Sorry I didn't handle it. Just hope to catch attentions from someone
(ie. you :).

It's not a problem for x86_64 at all. For others I wonder if any
driver will vmalloc HIGHMEM pages with !_PAGE_CACHE_WB attribute..

So I noted the possible problem and leave it alone.

> What's the problem with the current code, exactly? I would prefer that

- unnecessary complexity to handle multi-page case, since it's always
  called to access one single page;

- the kmap_atomic() cache consistency problem, which I expressed some
  concern (without further action)

> you continue using the same vmlist locking and checking for validating
> addresses.

It's a reasonable suggestion. Kame, would you agree on killing the
kmap_atomic() and revert to the vmlist walk?

Thanks,
Fengguang