malloc: Do not use MAP_NORESERVE to allocate heap segments
Address space for heap segments is reserved in a mmap call with MAP_ANONYMOUS | MAP_PRIVATE and protection flags PROT_NONE. This reservation does not count against the RSS limit of the process or system. Backing memory is allocated using mprotect in alloc_new_heap and grow_heap, and at this point, the allocator expects the kernel to provide memory (subject to memory overcommit). The SIGSEGV that might generate due to MAP_NORESERVE (according to the mmap manual page) does not seem to occur in practice, it's always SIGKILL from the OOM killer. Even if there is a way that SIGSEGV could be generated, it is confusing to applications that this only happens for secondary heaps, not for large mmap-based allocations, and not for the main arena. Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
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@ -559,16 +559,13 @@ new_heap (size_t size, size_t top_pad)
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#if HAVE_TUNABLES
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#if HAVE_TUNABLES
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if (__glibc_unlikely (mp_.hp_pagesize != 0))
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if (__glibc_unlikely (mp_.hp_pagesize != 0))
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{
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{
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/* MAP_NORESERVE is not used for huge pages because some kernel may
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not reserve the mmap region and a subsequent access may trigger
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a SIGBUS if there is no free pages in the pool. */
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heap_info *h = alloc_new_heap (size, top_pad, mp_.hp_pagesize,
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heap_info *h = alloc_new_heap (size, top_pad, mp_.hp_pagesize,
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mp_.hp_flags);
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mp_.hp_flags);
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if (h != NULL)
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if (h != NULL)
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return h;
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return h;
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}
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}
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#endif
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#endif
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return alloc_new_heap (size, top_pad, GLRO (dl_pagesize), MAP_NORESERVE);
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return alloc_new_heap (size, top_pad, GLRO (dl_pagesize), 0);
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}
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}
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/* Grow a heap. size is automatically rounded up to a
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/* Grow a heap. size is automatically rounded up to a
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@ -1110,10 +1110,6 @@ static mchunkptr mremap_chunk(mchunkptr p, size_t new_size);
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# define MAP_ANONYMOUS MAP_ANON
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# define MAP_ANONYMOUS MAP_ANON
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#endif
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#endif
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#ifndef MAP_NORESERVE
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# define MAP_NORESERVE 0
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#endif
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#define MMAP(addr, size, prot, flags) \
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#define MMAP(addr, size, prot, flags) \
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__mmap((addr), (size), (prot), (flags)|MAP_ANONYMOUS|MAP_PRIVATE, -1, 0)
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__mmap((addr), (size), (prot), (flags)|MAP_ANONYMOUS|MAP_PRIVATE, -1, 0)
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