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Kernel: Add MAP_NORESERVE support to mmap

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Rather than lazily committing regions by default, we now commit
the entire region unless MAP_NORESERVE is specified.

This solves random crashes in low-memory situations where e.g. the
malloc heap allocated memory, but using pages that haven't been
used before triggers a crash when no more physical memory is available.

Use this flag to create large regions without actually committing
the backing memory. madvise() can be used to commit arbitrary areas
of such regions after creating them.
This commit is contained in:
Tom 2020-09-03 15:06:25 -06:00 • committed by Andreas Kling
parent bc5d6992a4
commit c3451899bc
7 changed files with 30 additions and 15 deletions
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6
Kernel/Syscalls/mmap.cpp
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@ -120,6 +120,7 @@ void* Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> user_params)
bool map_private = flags & MAP_PRIVATE;
bool map_stack = flags & MAP_STACK;
bool map_fixed = flags & MAP_FIXED;
bool map_noreserve = flags & MAP_NORESERVE;
if (map_shared && map_private)
return (void*)-EINVAL;
@ -148,10 +149,9 @@ void* Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> user_params)
if (!region && (!map_fixed && addr != 0))
region = allocate_region_with_vmobject({}, size, vmobject, 0, !name.is_null() ? name : "mmap (purgeable)", prot);
} else if (map_anonymous) {
region = allocate_region(range.value(), !name.is_null() ? name : "mmap", prot, false);
region = allocate_region(range.value(), !name.is_null() ? name : "mmap", prot, !map_noreserve);
if (!region && (!map_fixed && addr != 0))
region = allocate_region(allocate_range({}, size), !name.is_null() ? name : "mmap", prot, false);
region = allocate_region(allocate_range({}, size), !name.is_null() ? name : "mmap", prot, !map_noreserve);
} else {
if (offset < 0)
return (void*)-EINVAL;