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 08:03:33AM +0300, Mika Penttila wrote:
> KAMEZAWA Hiroyuki wrote:
> >On Tue, 29 Apr 2008 10:10:58 -0400
> >Tony Battersby <tonyb@cybernetics.com> 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 ?
> >
> >(I'm sorry but I'll not be active for a week because my servers are 
> >powered off.)
> >
> >-Kame
> >
> >  
> 
> 
> But sure we still have to handle the fault for instance swapped pages, 
> for other uses of get_user_pages();

Yeah, it does need to test for pte_none.

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