[3.14] Doc/c-api/memory.rst: extend --without-pymalloc doc with ASan information (GH-136790) (GH-136798)

miss-islington · disconnect3d · encukou · web-flow · commit cce38eefd4b8 · 2025-07-20T10:55:06.000+02:00
Extend the documentation for disabling pymalloc with the `--without-pymalloc` flag regarding why it is worth to use it when enabling AddressSanitizer for Python build (which is done, e.g., in CPython's CI builds). I have tested the CPython latest main build with both ASan and pymalloc enabled and it seems to work just fine. I did run the `python -m test` suite which didn't uncover any ASan crashes (though, it detected some memory leaks, which I believe are irrelevant here). I have discussed ASan and this flag with @encukou on the CPython Core sprint on EuroPython 2025. We initially thought that the `--without-pymalloc` flag is needed for ASan builds due to the fact pymalloc must hit the begining of page when determining if the memory to be freed comes from pymalloc or was allocated by the system malloc. In other words, we thought, that ASan would crash CPython during free of big objects (allocated by system malloc). It may be that this was the case in the past, but it is not the case anymore as the `address_in_range` function used by pymalloc is annotated to be skipped from the ASan instrumentation. (cherry picked from commit d19bb44) Co-authored-by: Disconnect3d <dominik.b.czarnota@gmail.com> Co-authored-by: Petr Viktorin <encukou@gmail.com>
diff --git a/Doc/c-api/memory.rst b/Doc/c-api/memory.rst
@@ -672,6 +672,10 @@ This allocator is disabled if Python is configured with the
 :option:`--without-pymalloc` option. It can also be disabled at runtime using
 the :envvar:`PYTHONMALLOC` environment variable (ex: ``PYTHONMALLOC=malloc``).
 
+Typically, it makes sense to disable the pymalloc allocator when building
+Python with AddressSanitizer (:option:`--with-address-sanitizer`) which helps
+uncover low level bugs within the C code.
+
 Customize pymalloc Arena Allocator
 ----------------------------------
 
diff --git a/Doc/using/configure.rst b/Doc/using/configure.rst
@@ -802,6 +802,9 @@ Debug options
 .. option:: --with-address-sanitizer
 
    Enable AddressSanitizer memory error detector, ``asan`` (default is no).
+   To improve ASan detection capabilities you may also want to combine this
+   with :option:`--without-pymalloc` to disable the specialized small-object
+   allocator whose allocations are not tracked by ASan.
 
    .. versionadded:: 3.6
 
