New submission from STINNER Victor <[email protected]>:
Objects/obmalloc.c contains an interesting comment:
if (!address_in_range(p, pool)) {
/* pymalloc is not managing this block.
If nbytes <= SMALL_REQUEST_THRESHOLD, it's tempting to try to take
over this block. However, if we do, we need to copy the valid data
from the C-managed block to one of our blocks, and there's no
portable way to know how much of the memory space starting at p is
valid.
As bug 1185883 pointed out the hard way, it's possible that the
C-managed block is "at the end" of allocated VM space, so that a
memory fault can occur if we try to copy nbytes bytes starting at p.
Instead we punt: let C continue to manage this block. */
return 0;
}
See also bpo-1185883.
We don't have to guess, it's possible to get the size of a memory block
allocated by malloc() with:
* malloc_usable_size(ptr): available at least on Linux
* _msize(ptr): Windows
Maybe we could add a "msize" field to PyMemAllocatorEx? See also bpo-18835 whic
adds PyMem_AlignedAlloc() and so already modify (and rename PyMemAllocatorEx).
See also bpo-31626, but I'm not sure that it would help for
_PyMem_DebugRawRealloc().
----------
components: Interpreter Core
messages: 305319
nosy: haypo, serhiy.storchaka, skrah
priority: normal
severity: normal
status: open
title: Use malloc_usable_size() is pymalloc for realloc
type: performance
versions: Python 3.7
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<https://bugs.python.org/issue31911>
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