gnu/packages/patches/ghc-8.0-fall-back-to-madv_dontneed.patch

   1 ghc runtime by default (otherwise depending on a "configure" option)
   2 does memory allocation on their own by first mmapping a 1 TB range of
   3 memory into the process and then parceling out chunks from it.
   4
   5 If one of the chunks is not needed, the kernel needs to be informed -
   6 otherwise the system would quickly run out of available RAM.
   7
   8 ghc does that via madvise(2).
   9
  10 There are two options when doing this informing:
  11
  12 MADV_FREE - Means "I don't need this range or the data in it any more".
  13 Kernel promises to fail later accesses to it.
  14
  15 MADV_DONTNEED - Means "I don't need this range right now - and I don't
  16 need the data in it anymore". Kernel promises to make later accesses to
  17 it succeed (if necessary by providing a new page initialized with zeroes).
  18
  19 MADV_FREE was introduced in Linux 4.5.
  20 glibc 2.25 and later always define MADV_FREE.
  21
  22 Unpatched ghc 8.0.2 will use either MADV_FREE or MADV_DONTNEED, determined
  23 at ghc compile time.  Which of them will actually succeed is determined
  24 by the Linux kernel at run time.
  25
  26 This patch makes ghc try MADV_FREE.  If it doesn't work, it falls back to
  27 MADV_DONTNEED.
  28
  29 The end result is that ghc programs free their memory with Linux < 4.5 again.
  30
  31 See https://git.haskell.org/ghc.git/commitdiff/6576bf83cdf4eac05eb88a24aa934a736c91e3da for more information.
  32 --- a/rts/posix/OSMem.c
  33 +++ b/rts/posix/OSMem.c
  34 @@ -541,11 +541,24 @@ void osDecommitMemory(void *at, W_ size)
  35
  36  #ifdef MADV_FREE
  37      // Try MADV_FREE first, FreeBSD has both and MADV_DONTNEED
  38 -    // just swaps memory out
  39 +    // just swaps memory out. Linux >= 4.5 has both DONTNEED and FREE; either
  40 +    // will work as they both allow the system to free anonymous pages.
  41 +    // It is important that we try both methods as the kernel which we were
  42 +    // built on may differ from the kernel we are now running on.
  43      r = madvise(at, size, MADV_FREE);
  44 -#else
  45 -    r = madvise(at, size, MADV_DONTNEED);
  46 +    if(r < 0) {
  47 +        if (errno == EINVAL) {
  48 +            // Perhaps the system doesn't support MADV_FREE; fall-through and
  49 +            // try MADV_DONTNEED.
  50 +        } else {
  51 +            sysErrorBelch("unable to decommit memory");
  52 +        }
  53 +    } else {
  54 +        return;
  55 +    }
  56  #endif
  57 +
  58 +    r = madvise(at, size, MADV_DONTNEED);
  59      if(r < 0)
  60          sysErrorBelch("unable to decommit memory");
  61  }