Re: unuse_pte: set pte dirty if the page is dirty

[Andrew Morton <akpm@osdl.org>]

Christoph Lameter <clameter@engr.sgi.com> wrote:
>
> When replacing a swap pte with a real pte in unuse_pte, we simply generate
> a pte that has no dirty bit set regardless of what state the page is in.
> 
> If a process wants to write to a dirty page after replacement then a
> page fault has to first  set the dirty bit in the pte.
> 
> This patch generates a pte with the dirty bit already set and so avoids
> that fault.
> 
> Page migration moves a page from regular ptes to swap ptes and back
> for anonymous page and so may generate lots of ptes that are not marked
> dirty. This patch will increase the efficiency of page migration.
> 
> Signed-off-by: Christoph Lameter <clameter@sgi.com>
> 
> Index: linux-2.6.16-rc5/mm/swapfile.c
> ===================================================================
> --- linux-2.6.16-rc5.orig/mm/swapfile.c	2006-02-26 21:09:35.000000000 -0800
> +++ linux-2.6.16-rc5/mm/swapfile.c	2006-02-27 17:17:38.000000000 -0800
> @@ -425,10 +425,14 @@ void free_swap_and_cache(swp_entry_t ent
>  static void unuse_pte(struct vm_area_struct *vma, pte_t *pte,
>  		unsigned long addr, swp_entry_t entry, struct page *page)
>  {
> +	pte_t new_pte = pte_mkold(mk_pte(page, vma->vm_page_prot));
> +
>  	inc_mm_counter(vma->vm_mm, anon_rss);
>  	get_page(page);
> +
>  	set_pte_at(vma->vm_mm, addr, pte,
> -		   pte_mkold(mk_pte(page, vma->vm_page_prot)));
> +			PageDirty(page) ? pte_mkdirty(new_pte) : new_pte);
> +
>  	page_add_anon_rmap(page, vma, addr);
>  	swap_free(entry);
>  	/*

Are we sure this is race-free?  Say, someone is in the process of cleaning
the page?  munmap, conceivably swapout?  We end up with a dirty pte
pointing at a now-clean page.  The page will later become dirty again.  Is
that a problem?  It would generate a surprise if the vma had ben set
read-only in the interim, for example.

I can't immediately see a problem, but haven't thought about it a lot..

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