Virtual copying of FastMalloc allocated memory causes madvise MADV_FREE_REUSABLE errors

• Component: WebCore Misc.
• Status: RESOLVED FIXED

[Michael Saboff]

There are two instances where we make virtual copies of memory allocated via FastMalloc.  One is when we use vm_copy in PurgeableBuffer::create() and the other is implicit in CoreIPC where we use the MACH_MSG_VIRTUAL_COPY flag in Connection::sendOutgoingMessage.  In both cases, the kernel creates a second reference to the region being copied.  These additional references preclude using the MADV_FREE_REUSABLE flag in madvise.  This stops us from making memory available to the system for other processes.

[Michael Saboff]

Created attachment 102602 [details]
Proposed patch

[Michael Saboff]

Two defects in Radar: <rdar://problem/9747241> and <rdar://problem/9747279>.

[Darin Adler]

Comment on attachment 102602 [details]
Proposed patch

View in context: https://bugs.webkit.org/attachment.cgi?id=102602&action=review

> Source/WebCore/platform/mac/PurgeableBufferMac.cpp:67
> +    memcpy(reinterpret_cast<char*>(buffer), data, size);

Since memcpy’s destination is a void* I would not expect you’d need to typecast a pointer to use it as a memcpy destination. Can you just pass buffer without a typecast?

[Michael Saboff]

(In reply to comment #3)
> (From update of attachment 102602 [details])
> View in context: https://bugs.webkit.org/attachment.cgi?id=102602&action=review
> 
> > Source/WebCore/platform/mac/PurgeableBufferMac.cpp:67
> > +    memcpy(reinterpret_cast<char*>(buffer), data, size);
> 
> Since memcpy’s destination is a void* I would not expect you’d need to typecast a pointer to use it as a memcpy destination. Can you just pass buffer without a typecast?

The cast is needed because vm_address_t is really a uintptr_t which is an unsigned long, a non-pointer type.

[Michael Saboff]

Committed r92231: <http://trac.webkit.org/changeset/92231>