[mimalloc] Define MI_FREE_USE_PAGEMAP

[kleisauke]

Upstream commit microsoft/mimalloc@0d17826 renamed MI_OPT_FREE to MI_FREE_USE_PAGEMAP, with its meaning reversed.

As we did not previously define MI_OPT_FREE, define MI_FREE_USE_PAGEMAP instead. This also matches the upstream configuration for WASI targets.

Fixes: #27978

[kleisauke]

This also matches the upstream configuration for WASI targets.

See: https://github.com/microsoft/mimalloc/blob/v3.5.4/CMakeLists.txt#L545-L548.

[juj]

Nice - glad it turned out to be fixable via a config change.

[sbc100]

Can you add a a little detail in the PR description as to what the crash in #27978 and how this fixes it?

[sbc100]

lgtm % comment

[sbc100]

Also, can you add wasmfs.test_wrap_malloc_mimalloc to the subset of wasmfs tests we run in circleci?

[kleisauke]

I think this landed as-is due to auto-merge. The commit message is slightly wrong, as I incorrectly assumed that MI_OPT_FREE was available in mimalloc 3.4.1 (i.e. the version we used before commit 9eca018 landed)

Can you add a a little detail in the PR description as to what the crash in #27978 and how this fixes it?

I'm not exactly sure why this fixes #27978 (or why increasing -sINITIAL_MEMORY works around the issue). This change effectively switches us from a 2-level page map to a flat one. I suspect the 2-level map may not work correctly with Emscripten's default INITIAL_HEAP of 16 MiB. For reference, see:

emscripten/system/lib/mimalloc/include/mimalloc/bits.h

Lines 139 to 146 in 61b8d61

	// use a flat page-map or a 2-level one
	#ifndef MI_PAGE_MAP_FLAT
	#if MI_MAX_VABITS <= 40 && !defined(__APPLE__) && !MI_SECURE && !MI_FREE_IS_CHECKED && MI_FREE_USE_PAGEMAP
	#define MI_PAGE_MAP_FLAT 1
	#else
	#define MI_PAGE_MAP_FLAT 0
	#endif
	#endif

and:

emscripten/system/lib/mimalloc/include/mimalloc/internal.h

Lines 673 to 675 in 61b8d61

	// flat page-map committed on demand, using one byte per slice (64 KiB).
	// single indirection and low commit, but large initial virtual reserve (4 GiB with 48 bit virtual addresses)
	// used by default on <= 40 bit virtual address spaces.

versus:

emscripten/system/lib/mimalloc/include/mimalloc/internal.h

Lines 711 to 717 in 61b8d61

	// 2-level page map:
	// double indirection, but low commit and low virtual reserve.
	//
	// the page-map is usually 4 MiB (for 48 bit virtual addresses) and points to sub maps of 64 KiB.
	// the page-map is committed on-demand (in 64 KiB parts) (and sub-maps are committed on-demand as well)
	// one sub page-map = 64 KiB => covers 2^(16-3) * 2^16 = 2^29 = 512 MiB address space
	// the page-map needs 48-(16+13) = 19 bits => 2^19 sub map pointers = 2^22 bytes = 4 MiB reserved size.

(before commit 9eca018, MI_PAGE_MAP_FLAT was also implicitly set to 1)

Feel free to revert and investigate further, though debugging allocator issues can be very tricky.

[sbc100]

I think just folloing up with adding wasmfs.test_wrap_malloc_mimalloc to circleci is fine