Re: [rfc][patch 3/5] afs: new aops

[Nick Piggin <npiggin@suse.de>]

On Wed, Nov 14, 2007 at 03:57:46PM +0000, David Howells wrote:
> Nick Piggin <npiggin@suse.de> wrote:
> 
> > In core code, the PAGE_CACHE_SIZE is for page cache struct pages. Single
> > struct pages (not page arrays).  Take a look at generic mapping read or
> > something.
> 
> So you're saying a struct page controls an area of PAGE_CACHE_SIZE, not an
> area of PAGE_SIZE?

No, a pagecache page is PAGE_CACHE_SIZE. And not all struct pages control
the same amount of data anyway, with compound pages.


> Is a struct page then a purely pagecache concept?
> 
> > Documentation is the opposite of convention ;)
> 
> If it's not Documented, then it's irrelevant.

But you can't just decide yourself that it is irrelevant because you don't
grep hard enough ;)

include/linux/mm.h:
 * A page may belong to an inode's memory mapping. In this case, page->mapping
 * is the pointer to the inode, and page->index is the file offset of the page,
 * in units of PAGE_CACHE_SIZE.

include/linux/mm_types.h
        unsigned long vm_pgoff;         /* Offset (within vm_file) in PAGE_SIZE
                                           units, *not* PAGE_CACHE_SIZE */

Looks like documentation to me.