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

    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;
    + }
    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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