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    SubjectRe: pipe/page fault oddness.
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    On Mon, Sep 29, 2014 at 8:33 PM, Dave Jones <davej@redhat.com> wrote:
    >
    > Looking at the dump, there's only one running trinity child,
    > with all the others blocking on it.
    >
    > trinity-c49 R running task 12856 19464 7633 0x00000004
    > ffff8800a09bf960 0000000000000002 ffff8800a09bf9f8 ffff880219650000
    > 00000000001d4080 0000000000000000 ffff8800a09bffd8 00000000001d4080
    > ffff88023f755bc0 ffff880219650000 ffff8800a09bffd8 ffff88010b017e00
    > Call Trace:
    > [<ffffffff911c3c67>] handle_mm_fault+0x3a7/0xcd0
    > [<ffffffff91042c84>] __do_page_fault+0x1a4/0x600
    > [<ffffffff910430fe>] do_page_fault+0x1e/0x70
    > [<ffffffff918264b2>] page_fault+0x22/0x30
    > [<ffffffff911bd7e3>] ? copy_page_to_iter+0x3b3/0x500
    > [<ffffffff9120eddf>] pipe_read+0xdf/0x330
    >
    > Running the function tracer on that pid shows it spinning forever..
    > http://codemonkey.org.uk/junk/pipe-trace.txt
    >
    > Kernel bug (missing EFAULT check somewhere perhaps?), or is this a
    > case where the fuzzer asked the kernel to do something stupid, and it obliged ?

    Hmm. It looks like copy_page_to_iter_iovec() is broken and keeps not
    making any progress while just faulting.

    I don't see how that could happen, though. All the loops there are
    conditional on the user copies *not* failing (ie "!left"), and they
    seem to properly update "iov".

    Mind sending a disassembly of your "copy_page_to_iter" function, in
    particular around that whole "0x3b3/0x500" area which is where the
    page fault seems to happen?

    Adding Al to the cc, since this code is from his commit 6e58e79db8a1
    ("introduce copy_page_to_iter, kill loop over iovec in
    generic_file_aio_read()") but I don't see anything obviously wrong
    there.

    Al? Do you see something I don't? Dave's function trace does seem to
    say that it doesn't even get back to pipe_read(), though, so the loop
    really must be inside copy_page_to_iter().

    Linus


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