Re: [PATCH] more ZERO_PAGE handling ( was 2.6.24 regression: deadlock on coredump of big process)

[Nick Piggin <npiggin@suse.de>]

On Wed, Apr 30, 2008 at 02:35:42PM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 30 Apr 2008 07:19:32 +0200
> Nick Piggin <npiggin@suse.de> wrote:
> 
> > 
> > Something like this should do:
> > if (!pte_present(pte)) {
> > 	if (pte_none(pte)) {
> > 		pte_unmap_unlock
> > 		goto null_or_zeropage;
> > 	}
> > 	goto unlock;
> > }
> > 
> Sorry for broken work and thank you for advice.
> updated.

Don't be sorry. The most important thing is that you found this tricky
problem. That's very good work IMO!

> 
> Regards,
> -Kame
> ==
> follow_page() returns ZERO_PAGE if a page table is not available.
> but returns NULL if a page table exists. If NULL, handle_mm_fault()
> allocates a new page.
> 
> This behavior increases page consumption at coredump, which tend
> to do read-once-but-never-written page fault.  This patch is
> for avoiding this.
> 
> Changelog:
>   - fixed to check pte_present()/pte_none() in proper way.
> 
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> 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)))
> -		goto no_page_table;
> +		goto null_or_zeropage;
>  
>  	if (pmd_huge(*pmd)) {
>  		BUG_ON(flags & FOLL_GET);
> @@ -947,8 +947,13 @@ struct page *follow_page(struct vm_area_
>  		goto out;
>  
>  	pte = *ptep;
> -	if (!pte_present(pte))
> +	if (!pte_present(pte)) {
> +		if (!(flags & FOLL_WRITE) && pte_none(pte)) {
> +			pte_unmap_unlock(ptep, ptl);
> +			goto null_or_zeropage;
> +		}
>  		goto unlock;
> +	}

Just a small nitpick: I guess you don't need this FOLL_WRITE test because
null_or_zeropage will test FOLL_ANON which implies !FOLL_WRITE. It should give
slightly smaller code.

Otherwise, looks good to me:

Acked-by: Nick Piggin <npiggin@suse.de>


>  	if ((flags & FOLL_WRITE) && !pte_write(pte))
>  		goto unlock;
>  	page = vm_normal_page(vma, address, pte);
> @@ -968,7 +973,7 @@ unlock:
>  out:
>  	return page;
>  
> -no_page_table:
> +null_or_zeropage:
>  	/*
>  	 * When core dumping an enormous anonymous area that nobody
>  	 * has touched so far, we don't want to allocate page tables.

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