Re: [PATCH] Add PAGE_CACHE_PAGES

[Hugh Dickins]

On 15 Aug 2002, Gilad Ben-Yossef wrote:
> 
> Thus spake mm/shmem.c:
> 
>   idx = (address - vma->vm_start) >> PAGE_CACHE_SHIFT;
>   idx += vma->vm_pgoff;
> 
> But include/linux/mm.h says:
> 
> unsigned long vm_pgoff;         /* Offset (within vm_file) in PAGE_SIZE
>                                    units, *not* PAGE_CACHE_SIZE */

Good observation, but it's certainly not something for Marcelo to worry
about for 2.4.  This is only one of many confusions which would need to
be fixed to get PAGE_CACHE_SIZE > PAGE_SIZE working.  mm/shmem.c
is probably especially confused (since it's backing a PAGE_CACHE_SIZE-
orientated filesystem with PAGE_SIZE-orientated swap), but much else too.

I think it was a mistake to introduce the PAGE_CACHE_SIZE definition
before it could be allowed to diverge from PAGE_SIZE: I find it very
hard to work out the right way to go, and would prefer it to vanish. 
But Ben LaHaise did succeed in getting a PAGE_CACHE_SIZE > PAGE_SIZE
patch working on 2.4.6 (no patch to mm/shmem.c, but it was a little
different then anyway, right or wrong I don't know).

Hugh