RE: [PATCHv12 2/4] zbud: add to mm/

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Andrew Morton [mailto:akpm@linux-foundation.org]
> Subject: Re: [PATCHv12 2/4] zbud: add to mm/
> 
> On Wed, 29 May 2013 15:42:36 -0500 Seth Jennings <sjenning@linux.vnet.ibm.com> wrote:
> 
> > > > > I worry about any code which independently looks at the pageframe
> > > > > tables and expects to find page struts there.  One example is probably
> > > > > memory_failure() but there are probably others.
> > >
> > > ^^ this, please.  It could be kinda fatal.
> >
> > I'll look into this.
> >
> > The expected behavior is that memory_failure() should handle zbud pages in the
> > same way that it handles in-use slub/slab/slob pages and return -EBUSY.
> 
> memory_failure() is merely an example of a general problem: code which
> reads from the memmap[] array and expects its elements to be of type
> `struct page'.  Other examples might be memory hotplugging, memory leak
> checkers etc.  I have vague memories of out-of-tree patches
> (bigphysarea?) doing this as well.
> 
> It's a general problem to which we need a general solution.

<Obi-tmem Kenobe slowly materializes... "use the force, Luke!">

One could reasonably argue that any code that makes incorrect
assumptions about the contents of a struct page structure is buggy
and should be fixed.  Isn't the "general solution" already described
in the following comment, excerpted from include/linux/mm.h, which
implies that "scribbling on existing pageframes" [carefully], is fine?
(And, if not, shouldn't that comment be fixed, or am I misreading
it?)

<start excerpt>
 * For the non-reserved pages, page_count(page) denotes a reference count.
 *   page_count() == 0 means the page is free. page->lru is then used for
 *   freelist management in the buddy allocator.
 *   page_count() > 0  means the page has been allocated.
 *
 * Pages are allocated by the slab allocator in order to provide memory
 * to kmalloc and kmem_cache_alloc. In this case, the management of the
 * page, and the fields in 'struct page' are the responsibility of mm/slab.c
 * unless a particular usage is carefully commented. (the responsibility of
 * freeing the kmalloc memory is the caller's, of course).
 *
 * A page may be used by anyone else who does a __get_free_page().
 * In this case, page_count still tracks the references, and should only
 * be used through the normal accessor functions. The top bits of page->flags
 * and page->virtual store page management information, but all other fields
 * are unused and could be used privately, carefully. The management of this
 * page is the responsibility of the one who allocated it, and those who have
 * subsequently been given references to it.
 *
 * The other pages (we may call them "pagecache pages") are completely
 * managed by the Linux memory manager: I/O, buffers, swapping etc.
 * The following discussion applies only to them.
<end excerpt>

<Obi-tmem Kenobe slowly dematerializes>

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