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mmap: Use MAP_NORESERVE to request overcommit, not PROT_NONE.

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This reverts hrev54120 and instead adds the commonly supported
MAP_NORESERVE flag to request overcommit.

Using PROT_NONE for overcommit is problematic as the protection of
individual pages can still be changed via mprotect to make them
accessible, but that won't change the commitment. An application
using such a pattern may then unexpectedly run into out of memory
conditions on random writes into the address space.

With MAP_NORESERVE the overcommit can explicitly be requested by
applications that want to reserve address space without producing
memory pressure.

Change-Id: Id213d2245c5e23103e8e0857f7902e0cd8a2c65d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2611
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
This commit is contained in:
Michael Lotz 2020-05-08 22:57:07 +02:00 committed by waddlesplash
parent 7362b02b0e
commit b94221f3b2
2 changed files with 4 additions and 2 deletions
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1
headers/posix/sys/mman.h
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@ -23,6 +23,7 @@
#define MAP_FIXED 0x04 /* require mapping to specified addr */
#define MAP_ANONYMOUS 0x08 /* no underlying object */
#define MAP_ANON MAP_ANONYMOUS
#define MAP_NORESERVE 0x10 /* don't commit memory */
/* mmap() error return code */
#define MAP_FAILED ((void*)-1)

5
src/system/libroot/posix/sys/mman.cpp
View File

@ -131,8 +131,9 @@ mmap(void* address, size_t length, int protection, int flags, int fd,
areaProtection |= B_WRITE_AREA;
if ((protection & PROT_EXEC) != 0)
areaProtection |= B_EXECUTE_AREA;
if (protection == PROT_NONE)
areaProtection = B_OVERCOMMITTING_AREA;
if ((flags & MAP_NORESERVE) != 0)
areaProtection |= B_OVERCOMMITTING_AREA;
// create a name for this area based on calling image
void* addr = __builtin_return_address(0);