Brian Fundakowski Feldman wrote:
On Fri, May 20, 2005 at 01:40:35PM +0200, Hans Petter Selasky wrote:
Hi,
I just hit some problems with the new "contigmalloc()" routine in FreeBSD-6-current, which is used by "bus_dmamem_alloc()".
Firstly it locks Giant, which cause locking order reversals when allocating memory from certain contexts. Secondly it sleeps even if flag M_NOWAIT is passed. Thirdly it does not support flag M_ZERO.
Read the documentation. It supports M_ZERO just fine, and it does _not_ support M_NOWAIT.
Can someone explain why these changes have been made?
Changes?
Why doesn't "contigmalloc()" give a warning when an invalid flag is passed?
The kernel would be significantly larger and almost certainly slower if it were to double-check that everywhere any bit fields are used that flags that are not defined to have any behavior are unset.
Are these bugs in "contigmalloc()"?
No.
Or does the code using "contigmalloc()" have to be changed?
Yes. The i386 bus_dmamem_alloc(), for example, calls it with M_NOWAIT which is not a valid flag. The contigmalloc(9) page is not entirely truthful about the fact that it doesn't sleep at all -- it calls those routines which can. They can both be documented to require no locks to be held when being called, except for M_NOWAIT specifically in the one-page-or-less allocation case.
The amount of colateral damage that has been inflicted on bus_dmamem_alloc by your changes to contigmalloc is truly impressive.
Since I've spent the better part of 8 months cleaning it up, please
let me know what else needs to be done so that there are no more surprises. And no, this is not my happy face.
Scott _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[EMAIL PROTECTED]"
