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    Subject[tip:perf/core] uprobes: Clean up and document write_opcode()-> lock_page(old_page)
    Commit-ID:  9f92448ceeea5326db7d114005a7e7ac03904edf
    Gitweb: http://git.kernel.org/tip/9f92448ceeea5326db7d114005a7e7ac03904edf
    Author: Oleg Nesterov <oleg@redhat.com>
    AuthorDate: Sun, 29 Jul 2012 20:22:20 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Mon, 30 Jul 2012 11:27:20 +0200

    uprobes: Clean up and document write_opcode()->lock_page(old_page)

    The comment above write_opcode()->lock_page(old_page) tells
    about the race with do_wp_page(). I don't really understand
    which exactly race it means, but afaics this lock_page() was not
    enough to close all races with do_wp_page().

    Anyway, since:

    77fc4af1b59d uprobes: Change register_for_each_vma() to take mm->mmap_sem for writing

    this code is always called with ->mmap_sem held for writing,
    so we can forget about do_wp_page().

    However, we can't simply remove this lock_page(), and the only
    (afaics) reason is __replace_page()->try_to_free_swap().

    Nothing in write_opcode() needs it, move it into
    __replace_page() and fix the comment.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Acked-by: Srikar Dronamraju <srikar.vnet.ibm.com>
    Cc: Anton Arapov <anton@redhat.com>
    Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    Link: http://lkml.kernel.org/r/20120729182220.GA20322@redhat.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    kernel/events/uprobes.c | 27 ++++++++++++++-------------
    1 files changed, 14 insertions(+), 13 deletions(-)

    diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
    index 23c562b7..5db150b 100644
    --- a/kernel/events/uprobes.c
    +++ b/kernel/events/uprobes.c
    @@ -139,10 +139,15 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
    struct mm_struct *mm = vma->vm_mm;
    spinlock_t *ptl;
    pte_t *ptep;
    + int err;

    + /* freeze PageSwapCache() for try_to_free_swap() below */
    + lock_page(page);
    +
    + err = -EAGAIN;
    ptep = page_check_address(page, mm, addr, &ptl, 0);
    if (!ptep)
    - return -EAGAIN;
    + goto unlock;

    get_page(kpage);
    page_add_new_anon_rmap(kpage, vma, addr);
    @@ -162,7 +167,10 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
    put_page(page);
    pte_unmap_unlock(ptep, ptl);

    - return 0;
    + err = 0;
    + unlock:
    + unlock_page(page);
    + return err;
    }

    /**
    @@ -216,15 +224,10 @@ retry:
    ret = -ENOMEM;
    new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vaddr);
    if (!new_page)
    - goto put_out;
    + goto put_old;

    __SetPageUptodate(new_page);

    - /*
    - * lock page will serialize against do_wp_page()'s
    - * PageAnon() handling
    - */
    - lock_page(old_page);
    /* copy the page now that we've got it stable */
    vaddr_old = kmap_atomic(old_page);
    vaddr_new = kmap_atomic(new_page);
    @@ -237,15 +240,13 @@ retry:

    ret = anon_vma_prepare(vma);
    if (ret)
    - goto unlock_out;
    + goto put_new;

    ret = __replace_page(vma, vaddr, old_page, new_page);

    -unlock_out:
    - unlock_page(old_page);
    +put_new:
    page_cache_release(new_page);
    -
    -put_out:
    +put_old:
    put_page(old_page);

    if (unlikely(ret == -EAGAIN))

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