Messages in this thread | | | Date | Wed, 05 Nov 2008 20:47:44 +0200 | From | Török Edwin <> | Subject | Re: /proc/pid/maps containg anonymous maps that have PROT_NONE |
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On 2008-11-05 19:56, Török Edwin wrote: > On 2008-11-05 18:12, Hugh Dickins wrote: > >>> >>> >> mmap PROT_NONE to reserve an arena, munmap to trim off top and bottom, >> mprotect to make areas read+writable, madvise 0x4 to say MADV_DONTNEED >> on some parts. gcc? Or the application itself (clamd) and its libs? >> >> >> > >> Why does it mmap too much then trim it down? Perhaps it's trying to >> minimize pagetable usage, perhaps it's internally convenient to base >> on rounded addresses, I don't know. >> >> But the mmap is there: just easily overlooked because of the way it >> munmaps too (with strace showing hex addresses but decimal sizes). >> > > I will get some stacktraces and figure out, know that I know which mmap > to look for (the one with MAP_NORESERVE). >
I found it, glibc: arena.c:669 does it, there's a comment explaining why:
/* If consecutive mmap (0, HEAP_MAX_SIZE << 1, ...) calls return decreasing addresses as opposed to increasing, new_heap would badly fragment the address space. In that case remember the second HEAP_MAX_SIZE part aligned to HEAP_MAX_SIZE from last mmap (0, HEAP_MAX_SIZE << 1, ...) call (if it is already aligned) and try to reuse it next time. We need no locking for it, as kernel ensures the atomicity for us - worst case we'll call mmap (addr, HEAP_MAX_SIZE, ...) for some value of addr in multiple threads, but only one will succeed. */
Anyway it is MAP_NORESERVE, and PROT_NONE so it doesn't waste physical or swap memory.
Best regards, --Edwin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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