From 9fab36eb583c0e585e83a01253299afed9ea9a11 Mon Sep 17 00:00:00 2001 From: Siddhesh Poyarekar Date: Tue, 25 Sep 2012 14:10:29 +0530 Subject: Shrink heap on linux when overcommit_memory == 2 Using madvise with MADV_DONTNEED to release memory back to the kernel is not sufficient to change the commit charge accounted against the process on Linux. It is OK however, when overcommit is enabled or is heuristic. However, when overcommit is restricted to a percentage of memory setting the contents of /proc/sys/vm/overcommit_memory as 2, it makes a difference since memory requests will fail. Hence, we do what we do with secure exec binaries, which is to call mmap on the region to be dropped with MAP_FIXED. This internally unmaps the pages in question and reduces the amount of memory accounted against the process. --- malloc/arena.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) (limited to 'malloc/arena.c') diff --git a/malloc/arena.c b/malloc/arena.c index f24e76c4f6..b209e3b7cd 100644 --- a/malloc/arena.c +++ b/malloc/arena.c @@ -19,6 +19,9 @@ #include +/* Get the implementation for check_may_shrink_heap. */ +#include + /* Compile-time constants. */ #define HEAP_MIN_SIZE (32*1024) @@ -621,9 +624,9 @@ shrink_heap(heap_info *h, long diff) new_size = (long)h->size - diff; if(new_size < (long)sizeof(*h)) return -1; - /* Try to re-map the extra heap space freshly to save memory, and - make it inaccessible. */ - if (__builtin_expect (__libc_enable_secure, 0)) + /* Try to re-map the extra heap space freshly to save memory, and make it + inaccessible. See malloc-sysdep.h to know when this is true. */ + if (__builtin_expect (check_may_shrink_heap (), 0)) { if((char *)MMAP((char *)h + new_size, diff, PROT_NONE, MAP_FIXED) == (char *) MAP_FAILED) -- cgit v1.2.3