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

    --Mika



    > ==
    > 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@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 +947,10 @@ 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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