Re: [PATCH] Check for compound pages in set_page_dirty()

[Hugh Dickins <hugh@veritas.com>]

On Mon, 23 Jul 2007, Christoph Lameter wrote:
> On Thu, 19 Jul 2007 18:35:17 +0100 (BST)
> Hugh Dickins <hugh@veritas.com> wrote:
> 
> > I didn't see any attention to set_page_dirty in Christoph's
> > Large Blocksize (variable page_cachesize) patches, but I expect
> > he'd also be wanting set_page_dirty to act on compound_head.
> 
> There is no need for special casing if set_page_dirty() is only
> ever called for page heads. Make sure that direct I/O does not call
> set_page_dirty for tail pages.
> 
> A VM_BUG_ON(PageTail(page)) could be used to make sure that this
> does not happen in the future?

I expect we could take that approach in the current kernel, yes
(though it would put those compound tests into the bio code that
Jens was preferring to remove).  But I think not if your variable
page_cache_size went in: imagine an mmap of the tail component page
of an order-1 page_cache_size page, and that pte only being dirtied:
wouldn't set_page_dirty on that page need to redirect to the head?

Hugh