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    SubjectRe: /proc/$pid/pagemap troubles
    On Tue, Jul 31, 2007 at 01:36:14PM -0700, Dave Hansen wrote:
    > Since the pagemap code has a little header on it to help describe the
    > format, I wrote a little c program to parse its output. I get some
    > strange results. If I do this:
    >
    > fd = open("/proc/1/pagemap", O_RDONLY);
    > count = read(fd, &endianness, 1);
    >
    > count will always be 4.

    Known bug, fixed in my pending and not-currently-working update. It
    ought to return 0 for short reads.

    > hexdump gets similar, but even worse results:
    >
    > qemu:~# strace hexdump -C /proc/self/pagemap
    > ...
    > read(0, "\1\f\4\4\377\377\377\377\377\377\377\377\377\377\377\377"..., 16) = 20
    > read(0, 0x804d39c, 4294967292) = -1 EFAULT (Bad address)
    > --- SIGSEGV (Segmentation fault) @ 0 (0) ---
    > +++ killed by SIGSEGV +++
    >
    > Note that the kernel returns 20 to the read request of 16. I think the
    > kernel is actually copying over something important in hexdump's memory
    > which is adjacent to the buffer and causing it to segfault.

    Also fixed.

    > The code is basically organized not to output the right thing for any
    > unaligned access, and it apparently gets confused about exactly what
    > userspace has asked for. I think this is largely due to its overwriting
    > of "count" in pagemap_read().
    >
    > So, a couple of questions. Don't we need to support non-sizeof(unsigned
    > long)-aligned reads?

    Why? We should obviously never return more data than we were asked for
    (that's clearly a bug), but lots of things refuse to read or write
    stuff that isn't well sized and aligned.

    > Do we _really_ need that header in each and every file?

    Well there's either a header or there isn't.

    > > * first byte: 0 for big endian, 1 for little
    >
    > Do we ever have cases where userspace and kernel differ in their
    > endianness? Or, are you hoping to dump these files raw on one
    > architecture and parse them on another?

    Potentially, yes.

    > > * second byte: page shift (eg 12 for 4096 byte pages)
    >
    > This might actually (in theory) change on a per-process basis, so it
    > makes sense. But, it seems more global to the process that just pagemap
    > output. Would this always be the same as getpagesize()? Or, should it
    > always map 1:1 with the amount of memory mapped by a kernel pte_t. I
    > _think_ these can be slightly different because we have 64k PAGE_SIZE on
    > ppc64, but allow mappings to happen in 4k
    >
    > > * third byte: entry size in bytes (currently either 4 or 8)
    >
    > This one really boils down to "what is the kernel's sizeof(unsigned
    > long)" because we'll always store pfns in those. It seems like we
    > should have a better way to go fetch that.
    >
    > > * fourth byte: header size
    >
    > If we can get rid of the other three this, of course, goes away.

    True. But the variable-sized header lets us add other stuff later.

    --
    Mathematics is the supreme nostalgia of our time.
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