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    SubjectRe: [PATCH] mlock: revert the optimization for dirtying pages and triggering writeback.
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    On Sat, Jan 29, 2011 at 11:15 PM, Tao Ma <tm@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

    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 |
    VM_SHARED)) != VM_SHARED)
    gup_flags |= FOLL_WRITE;

    --
    Michel "Walken" Lespinasse
    A program is never fully debugged until the last user dies.
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