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Make smallbuddy handle larger requests correctly #556

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merged 4 commits into from
Sep 14, 2022
Merged

Make smallbuddy handle larger requests correctly #556

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07bb321
Fail more abruptly if the bounds are not exact.
mjp41 Sep 13, 2022
53c1e69
Move bounding from Pool into Backend.
mjp41 Sep 14, 2022
f83ade2
Enable smallbuddyrange to handle larger requests
mjp41 Sep 14, 2022
fcc3baf
Test larger requests for meta-data.
mjp41 Sep 14, 2022
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4 changes: 4 additions & 0 deletions src/snmalloc/aal/aal_cheri.h
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Original file line number Diff line number Diff line change
Expand Up @@ -86,6 +86,10 @@ namespace snmalloc
}

void* pb = __builtin_cheri_bounds_set_exact(a.unsafe_ptr(), size);

SNMALLOC_ASSERT(
__builtin_cheri_tag_get(pb) && "capptr_bound exactness failed.");

return CapPtr<T, BOut>::unsafe_from(static_cast<T*>(pb));
}

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8 changes: 6 additions & 2 deletions src/snmalloc/backend/backend.h
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Original file line number Diff line number Diff line change
Expand Up @@ -39,7 +39,7 @@ namespace snmalloc
* does not avail itself of this degree of freedom.
*/
template<typename T>
static capptr::Arena<void>
static capptr::Alloc<void>
alloc_meta_data(LocalState* local_state, size_t size)
{
capptr::Arena<void> p;
Expand All @@ -61,9 +61,13 @@ namespace snmalloc
}

if (p == nullptr)
{
errno = ENOMEM;
return nullptr;
}

return p;
return capptr_to_user_address_control(
Aal::capptr_bound<void, capptr::bounds::AllocFull>(p, size));
}

/**
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30 changes: 18 additions & 12 deletions src/snmalloc/backend_helpers/smallbuddyrange.h
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Expand Up @@ -174,13 +174,22 @@ namespace snmalloc
base,
length,
[this](CapPtr<void, ChunkBounds> base, size_t align, bool) {
CapPtr<void, ChunkBounds> overflow =
buddy_small
.add_block(
base.template as_reinterpret<FreeChunk<ChunkBounds>>(), align)
.template as_reinterpret<void>();
if (overflow != nullptr)
parent.dealloc_range(overflow, bits::one_at_bit(MIN_CHUNK_BITS));
if (align < MIN_CHUNK_SIZE)
{
CapPtr<void, ChunkBounds> overflow =
buddy_small
.add_block(
base.template as_reinterpret<FreeChunk<ChunkBounds>>(),
align)
.template as_reinterpret<void>();
if (overflow != nullptr)
parent.dealloc_range(
overflow, bits::one_at_bit(MIN_CHUNK_BITS));
}
else
{
parent.dealloc_range(base, align);
}
});
}

Expand All @@ -204,7 +213,8 @@ namespace snmalloc

CapPtr<void, ChunkBounds> alloc_range(size_t size)
{
SNMALLOC_ASSERT(size < MIN_CHUNK_SIZE);
if (size >= MIN_CHUNK_SIZE)
return parent.alloc_range(size);
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I recall us going back and forth on this previously, with the idea that the caller should skip the SmallBuddy ranges if they needed something larger. This fallback requires that the return type of the SmallBuddy match that of its parent, which might not, in general, be true: while a LargeBuddy "must" operate on CapPtrs intended to be large enough to include whole PageMap MetaEntrys worth of space, as it uses those for its metadata, the SmallBuddy, which uses in-band metadata, could have more tightly bounded types.

I suppose we're not actually taking advantage of that right now, tho'... but is there reason not to ask for the Range's parent in the caller instead?

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So ultimately we need a branch somewhere in alloc_meta_data. This would need to determine how large the request is and then which range to send it too. The code in SmallBuddyRange::alloc_with_leftover, returns the slop at the end of the allocation. This could be useful for large meta data allocations as well as small, so might want to be duplicated, though not essential.

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Following Teams call, I have added a test to ensure future changes don't break the Verona use case.

We will leave this design as is, but will revisit after @nwf-msr investigates a fine granularity of permissions.


auto result = buddy_small.remove_block(size);
if (result != nullptr)
Expand All @@ -218,8 +228,6 @@ namespace snmalloc

CapPtr<void, ChunkBounds> alloc_range_with_leftover(size_t size)
{
SNMALLOC_ASSERT(size < MIN_CHUNK_SIZE);

auto rsize = bits::next_pow2(size);

auto result = alloc_range(rsize);
Expand All @@ -236,8 +244,6 @@ namespace snmalloc

void dealloc_range(CapPtr<void, ChunkBounds> base, size_t size)
{
SNMALLOC_ASSERT(size < MIN_CHUNK_SIZE);

add_range(base, size);
}
};
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2 changes: 1 addition & 1 deletion src/snmalloc/mem/backend_concept.h
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Expand Up @@ -122,7 +122,7 @@ namespace snmalloc
{
Backend::template alloc_meta_data<void*>(local_state, size)
}
->ConceptSame<capptr::Arena<void>>;
->ConceptSame<capptr::Alloc<void>>;
}
&&requires(
LocalState& local_state,
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7 changes: 2 additions & 5 deletions src/snmalloc/mem/pool.h
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Expand Up @@ -137,11 +137,8 @@ namespace snmalloc
Config::Pal::error("Failed to initialise thread local allocator.");
}

p = capptr_to_user_address_control(
Aal::capptr_bound<T, capptr::bounds::AllocFull>(
capptr::Arena<T>::unsafe_from(new (raw.unsafe_ptr())
T(std::forward<Args>(args)...)),
sizeof(T)));
p = capptr::Alloc<T>::unsafe_from(new (raw.unsafe_ptr())
T(std::forward<Args>(args)...));

FlagLock f(pool.lock);
p->list_next = pool.list;
Expand Down
39 changes: 39 additions & 0 deletions src/test/func/pool/pool.cc
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Original file line number Diff line number Diff line change
@@ -1,3 +1,5 @@
#include <array>
#include <iostream>
#include <snmalloc/snmalloc.h>
#include <test/opt.h>
#include <test/setup.h>
Expand All @@ -24,6 +26,23 @@ struct PoolBEntry : Pooled<PoolBEntry>

using PoolB = Pool<PoolBEntry, Alloc::Config>;

struct PoolLargeEntry : Pooled<PoolLargeEntry>
{
std::array<int, 2'000'000> payload;

PoolLargeEntry()
{
printf(".");
fflush(stdout);
payload[0] = 1;
printf("first %d\n", payload[0]);
payload[1'999'999] = 1;
printf("last %d\n", payload[1'999'999]);
};
};

using PoolLarge = Pool<PoolLargeEntry, Alloc::Config>;

void test_alloc()
{
auto ptr = PoolA::acquire();
Expand Down Expand Up @@ -116,6 +135,18 @@ void test_iterator()
PoolA::release(after_iteration_ptr);
}

void test_large()
{
printf(".");
fflush(stdout);
PoolLargeEntry* p = PoolLarge::acquire();
printf(".");
fflush(stdout);
PoolLarge::release(p);
printf(".");
fflush(stdout);
}

int main(int argc, char** argv)
{
setup();
Expand All @@ -128,10 +159,18 @@ int main(int argc, char** argv)
#endif

test_alloc();
std::cout << "test_alloc passed" << std::endl;
test_constructor();
std::cout << "test_constructor passed" << std::endl;
test_alloc_many();
std::cout << "test_alloc_many passed" << std::endl;
test_double_alloc();
std::cout << "test_double_alloc passed" << std::endl;
test_different_alloc();
std::cout << "test_different_alloc passed" << std::endl;
test_iterator();
std::cout << "test_iterator passed" << std::endl;
test_large();
std::cout << "test_large passed" << std::endl;
return 0;
}