Fix: don't use overlapping mmap mappings on Cygwin
authorMichael Jeanson <mjeanson@efficios.com>
Fri, 28 Jul 2017 15:51:15 +0000 (11:51 -0400)
committerMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Tue, 7 Nov 2017 21:55:49 +0000 (16:55 -0500)
commit142af0ff11d9b4445a6229ccd22cd8fb4b38a798
tree18adefd7726baff76acead579440f6ca441889f5
parent748f9af1f0e61f6f5133b45ed7dcf1f071246b9e
Fix: don't use overlapping mmap mappings on Cygwin

The allocation scheme used by the mmap based RCU hash table is to make a
large unaccessible mapping to reserve memory without allocating it.
Then smaller chunks are allocated by overlapping read/write mappings which
do allocate memory. Deallocation is done by an overlapping unaccessible
mapping.

This scheme was tested on Linux, macOS and Solaris. However, on Cygwin the
mmap wrapper is based on the Windows NtMapViewOfSection API which doesn't
support overlapping mappings.

An alternative to the overlapping mappings is to use mprotect to change the
protection on chunks of the large mapping, read/write to allocate and none
to deallocate. This works perfecty on Cygwin and Solaris but on Linux a
call to madvise is also required to deallocate and it just doesn't work on
macOS.

For this reason, we keep to original scheme on all platforms except Cygwin.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
src/rculfhash-mm-mmap.c
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