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    SubjectRe: [PATCH] proc: put check_mem_permission before __get_free_page in mem_read
    Date
    Hi High,

    > On Sun, 17 Apr 2011, bookjovi@gmail.com wrote:
    > > From: Jovi Zhang <bookjovi@gmail.com>
    > >
    > > It should be better if put check_mem_permission before __get_free_page
    > > in mem_read, to be same as function mem_write.
    > >
    > > Signed-off-by: Jovi Zhang <bookjovi@gmail.com>
    >
    > Sorry to be contrary, but I disagree with this. I'm all for consistency,
    > but is there a particular reason why you think the mem_write ordering is
    > right and mem_read wrong?
    >
    > My reason for preferring the current mem_read ordering is this:
    >
    > check_mem_permission gets a reference to the mm. If we __get_free_page
    > after check_mem_permission, imagine what happens if the system is out
    > of memory, and the mm we're looking at is selected for killing by the
    > OOM killer: while we wait in __get_free_page for more memory, no memory
    > is freed from the selected mm because it cannot reach exit_mmap while
    > we hold that reference.

    Right.

    sorry for that. I missed this point.


    > (I may be overstating the case: a little memory may be freed from the
    > exiting task's stack, and kswapd should still be able to pick some pages
    > off the mm. But nonetheless, we would do better to let this mm go.)
    >
    > No doubt there are plenty of other places in /proc which try to
    > allocate memory after taking a reference on an mm; but I think
    > we should be working to eliminate those rather than add to them.

    then, Should we change mem_write instead?


    From 74f827ce74e1c4f846905e940edfa5f639b5a2ce Mon Sep 17 00:00:00 2001
    From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Date: Tue, 26 Apr 2011 13:57:02 +0900
    Subject: [PATCH] [PATCH] proc: put check_mem_permission after __get_free_page in mem_write

    It should be better if put check_mem_permission after __get_free_page
    in mem_write, to be same as function mem_read.

    Hugh Dickins explained the reason.

    check_mem_permission gets a reference to the mm. If we __get_free_page
    after check_mem_permission, imagine what happens if the system is out
    of memory, and the mm we're looking at is selected for killing by the
    OOM killer: while we wait in __get_free_page for more memory, no memory
    is freed from the selected mm because it cannot reach exit_mmap while
    we hold that reference.


    Reported-by: Jovi Zhang <bookjovi@gmail.com>
    Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Stephen Wilson <wilsons@start.ca>
    ---
    fs/proc/base.c | 16 +++++++++-------
    1 files changed, 9 insertions(+), 7 deletions(-)

    diff --git a/fs/proc/base.c b/fs/proc/base.c
    index 4deef2e..e93be6e 100644
    --- a/fs/proc/base.c
    +++ b/fs/proc/base.c
    @@ -894,20 +894,20 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
    if (!task)
    goto out_no_task;

    + copied = -ENOMEM;
    + page = (char *)__get_free_page(GFP_TEMPORARY);
    + if (!page)
    + goto out_task;
    +
    mm = check_mem_permission(task);
    copied = PTR_ERR(mm);
    if (IS_ERR(mm))
    - goto out_task;
    + goto out_free;

    copied = -EIO;
    if (file->private_data != (void *)((long)current->self_exec_id))
    goto out_mm;

    - copied = -ENOMEM;
    - page = (char *)__get_free_page(GFP_TEMPORARY);
    - if (!page)
    - goto out_mm;
    -
    copied = 0;
    while (count > 0) {
    int this_len, retval;
    @@ -929,9 +929,11 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
    count -= retval;
    }
    *ppos = dst;
    - free_page((unsigned long) page);
    +
    out_mm:
    mmput(mm);
    +out_free:
    + free_page((unsigned long) page);
    out_task:
    put_task_struct(task);
    out_no_task:
    --
    1.7.3.1




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