Date: Fri, 20 May 2005 10:39:12 -0600 From: Scott Long <scottl@samsco.org> To: Brian Fundakowski Feldman <green@freebsd.org> Cc: Hans Petter Selasky <hselasky@c2i.net> Subject: Re: problems with new the "contigmalloc" routine Message-ID: <428E12B0.9020208@samsco.org> In-Reply-To: <20050520151620.GF1201@green.homeunix.org> References: <200505201340.36176.hselasky@c2i.net> <20050520151620.GF1201@green.homeunix.org>
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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
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