Re: [PATCH] more ZERO_PAGE handling in follow_page()

[Andrew Morton]

On Wed, 7 May 2008 16:36:43 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> Rewrote the description of patch. (no changes in the logic.)
> 
> Thank you for all help.
> -Kame
> ==
> follow_page() is called from get_user_pages(), which returns specified user page.
> follow_page() can return 1) a page or 2) NULL or 3)ZERO_PAGE.
> If NULL, handle_mm_fault() is called.
> 
> Now, follow_page() to unused pte returns NULL if page table exists. As a result
> get_user_pages() calls handle_mm_fault() and allocate new memory.
> This behavior increases memory consumption at coredump, which does
> read-once-but-never-written page fault.
> By returning ZERO_PAGE() against READ/ANON request, we can avoid it.
> 
> (Because exec's arguments copy needs to call handle_mm_fault at WRITE/ANON
>  request, we just handle READ/ANON case here.)
> 
> Change log:
>   - Rewrote patch description and Added comments.
>   - fixed to check pte_present()/pte_none() in proper way.

So... how serious is the problem which we're fixing here?

I can see that if one is core-dumping large sparse address spaces this
could improve things a lot, but please help us understand the implications
so we can decide whether we need this in 2.6.26, thanks.


> Index: linux-2.6.25/mm/memory.c
> ===================================================================
> --- linux-2.6.25.orig/mm/memory.c
> +++ linux-2.6.25/mm/memory.c
> @@ -926,15 +926,15 @@ struct page *follow_page(struct vm_area_
>  	page = NULL;
>  	pgd = pgd_offset(mm, address);
>  	if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
> -		goto no_page_table;
> +		goto null_or_zeropage;
>  
>  	pud = pud_offset(pgd, address);
>  	if (pud_none(*pud) || unlikely(pud_bad(*pud)))
> -		goto no_page_table;
> +		goto null_or_zeropage;
>  	
>  	pmd = pmd_offset(pud, address);
>  	if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))

The mainline kernel does not have " || unlikely(pmd_bad(*pmd))" here. 
That got changed yesterday by

commit aeed5fce37196e09b4dac3a1c00d8b7122e040ce
Author: Hugh Dickins <hugh@xxxxxxxxxxx>
Date:   Tue May 6 20:49:23 2008 +0100

    x86: fix PAE pmd_bad bootup warning

So please confirm that the patch which I merged is still OK (I'd be
surprised if it isn't...)

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