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    SubjectRe: [PATCH] mlock: revert the optimization for dirtying pages and triggering writeback.
    On 01/30/2011 06:26 PM, Michel Lespinasse wrote:
    > On Sat, Jan 29, 2011 at 11:15 PM, Tao Ma<> wrote:
    >> buf = mmap(NULL, file_len, PROT_WRITE, MAP_SHARED, fd, 0);
    >> if (buf == MAP_FAILED) {
    >> perror("mmap");
    >> goto out;
    >> }
    >> if (mlock(buf, file_len)< 0) {
    >> perror("mlock");
    >> goto out;
    >> }
    > Thanks Tao for tracing this to an individual change. I can reproduce
    > this on my system. The issue is that the file is mapped without the
    > PROT_READ permission, so mlock can't fault in the pages. Up to 2.6.37
    > this worked because mlock was using a write.
    > The test case does show there was a behavior change; however it's not
    > clear to me that the tested behavior is valid.
    > I can see two possible resolutions:
    > 1- do nothing, if we can agree that the test case is invalid
    The test case does exist in the real world and used widespread. ;)
    It is blktrace.
    I can paste codes here also.
    In blktrace.c setup_mmap:
    mip->fs_buf = my_mmap(NULL, mip->fs_buf_len, PROT_WRITE,
    MAP_SHARED, fd,
    mip->fs_size - mip->fs_off);
    > 2- restore the previous behavior for writable, non-readable, shared
    > mappings while preserving the optimization for read/write shared
    > mappings. The test would then look like:
    > if ((vma->vm_flags& VM_WRITE)&& (vma->vm_flags& (VM_READ |
    > gup_flags |= FOLL_WRITE;
    I am not sure whether it is proper or not. I guess a fat comment is
    needed here
    to explain the corner case. So do you have some statistics that your change
    improve the performance a lot? If yes, I agree with you. Otherwise, I would
    prefer to revert it back to the original design.


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      Last update: 2011-01-30 14:59    [W:0.021 / U:14.976 seconds]
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