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

[Nick Piggin]

On Wed, Apr 30, 2008 at 01:25:16PM +0900, KAMEZAWA Hiroyuki wrote:
> On Tue, 29 Apr 2008 10:10:58 -0400
> Tony Battersby <tonyb@xxxxxxxxxxxxxxx> wrote:
> > 
> > If I leave more memory free by changing the argument to
> > malloc_all_but_x_mb(), then I have to increase the number of threads
> > required to trigger the deadlock.  Changing the thread stack size via
> > setrlimit(RLIMIT_STACK) also changes the number of threads that are
> > required to trigger the deadlock.  For example, with
> > malloc_all_but_x_mb(16) and the default stack size of 8 MB, <= 5 threads
> > will coredump successfully, and >= 6 threads will deadlock.  With
> > malloc_all_but_x_mb(16) and a reduced stack size of 4096 bytes, <= 8
> > threads will coredump successfully, and >= 9 threads will deadlock.
> > 
> > Also note that the "free" command reports 10 MB free memory while the
> > program is running before the segfault is triggered.
> > 
> Hmm, my idea is below.
> 
> Nick's remove ZERO_PAGE patch includes following change
> 
> ==
> @@ -2252,39 +2158,24 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
>         spinlock_t *ptl;
>  {
> <snip>
> -               page_add_new_anon_rmap(page, vma, address);
> -       } else {
> -               /* Map the ZERO_PAGE - vm_page_prot is readonly */
> -               page = ZERO_PAGE(address);
> -               page_cache_get(page);
> -               entry = mk_pte(page, vma->vm_page_prot);
> +       if (unlikely(anon_vma_prepare(vma)))
> +               goto oom;
> +       page = alloc_zeroed_user_highpage_movable(vma, address);
> ==
> 
> above change is for avoiding to use ZERO_PAGE at read-page-fault to anonymous
> vma. This is reasonable I think. But at coredump, tons of read-but-never-written 
> pages can be allocated.
> ==
> coredump
>   -> get_user_pages()
>        -> follow_page() returns NULL
>             -> handle mm fault
>                  -> do_anonymous page.
> ==
> follow_page() returns ZERO_PAGE only when page table is not avaiable.
> 
> So, making follow_page() return ZERO_PAGE can be a fix of extra memory
> consumpstion at core dump. (Maybe someone can think of other fix.)
> 
> how about this patch ? Could you try ?
> 

Ah, yes I stupidly missed this detail of follow_page. Definitely your
patch is a good idea, and I think it would be a good idea even when
we still had ZERO_PAGE, because it would prevent pagetable clearing
from having to do extra teardown work here.

Good catch, and I agree with your patch. Thanks


> (I'm sorry but I'll not be active for a week because my servers are powered off.)
> 
> -Kame
> 
> ==
> follow_page() returns ZERO_PAGE if page table is not available.
> but returns NULL pte is not presentl.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> 
> 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 +947,10 @@ struct page *follow_page(struct vm_area_
>  		goto out;
>  
>  	pte = *ptep;
> -	if (!pte_present(pte))
> -		goto unlock;
> +	if (!(flags & FOLL_WRITE) && !pte_present(pte)) {
> +		pte_unmap_unlock(ptep, ptl);
> +		goto null_or_zeropage;
> +	}
>  	if ((flags & FOLL_WRITE) && !pte_write(pte))
>  		goto unlock;
>  	page = vm_normal_page(vma, address, pte);
> @@ -968,7 +970,7 @@ 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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