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    SubjectRe: [PATCH] more ZERO_PAGE handling ( was 2.6.24 regression: deadlock on coredump of big process)
    KAMEZAWA Hiroyuki wrote:
    > 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;
    > + }
    > 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.
    >
    >
    >
    This patch fixes the deadlock. Tested on 2.6.24.5. Thanks!

    Tested-by: Tony Battersby <tonyb@cybernetics.com>

    Tony



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