Date: Fri, 30 May 2014 18:55:06 +0200 From: Attilio Rao <attilio@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: "src-committers@freebsd.org" <src-committers@freebsd.org>, Benno Rice <benno@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, Scott Long <scottl@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, "Peel, Casey" <casey.peel@isilon.com> Subject: Re: svn commit: r266775 - head/sys/x86/x86 Message-ID: <CAJ-FndCO=ydSk3rn0hu5_-jmQ4p=rCFnONe--OQQXM98PeX0=g@mail.gmail.com> In-Reply-To: <201405301244.07316.jhb@freebsd.org> References: <201405272131.s4RLVBEU035321@svn.freebsd.org> <201405301147.46872.jhb@freebsd.org> <CAJ-FndA_Z_52t2xD2=LftCcwuipsVUFNfGNM-1RZ4zu2NyGsJA@mail.gmail.com> <201405301244.07316.jhb@freebsd.org>
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On Fri, May 30, 2014 at 6:44 PM, John Baldwin <jhb@freebsd.org> wrote: > On Friday, May 30, 2014 11:51:38 am Attilio Rao wrote: >> On Fri, May 30, 2014 at 5:47 PM, John Baldwin <jhb@freebsd.org> wrote: >> > On Friday, May 30, 2014 11:39:24 am Attilio Rao wrote: >> >> On Fri, May 30, 2014 at 5:03 PM, John Baldwin <jhb@freebsd.org> wrote: >> >> > On Friday, May 30, 2014 10:54:06 am Attilio Rao wrote: >> >> >> On Tue, May 27, 2014 at 11:31 PM, Scott Long <scottl@freebsd.org> wrote: >> >> >> > Author: scottl >> >> >> > Date: Tue May 27 21:31:11 2014 >> >> >> > New Revision: 266775 >> >> >> > URL: http://svnweb.freebsd.org/changeset/base/266775 >> >> >> > >> >> >> > Log: >> >> >> > Eliminate the fake contig_dmamap and replace it with a new flag, >> >> >> > BUS_DMA_KMEM_ALLOC. They serve the same purpose, but using the flag >> >> >> > means that the map can be NULL again, which in turn enables significant >> >> >> > optimizations for the common case of no bouncing. >> >> >> >> >> >> While I think this is in general a good idea, unfortunately our >> >> >> drivers do so many dumb things when freeing DMA allocated buffers that >> >> >> having a NULL map is going to cause some "turbolence" and make such >> >> >> bugs more visible. >> >> >> An example is with ATA, where I think this fix is needed: >> >> >> http://www.freebsd.org/~attilio/dmamem_free-ata.patch >> >> >> >> >> >> Otherwise, what can happen with bounce buffers, is that the allocated >> >> >> memory via contig malloc was not going to be freed anytime. >> >> >> >> >> >> I tried to look around and I found questionable (read broken) code in >> >> >> basically every driver which allocates DMA buffers, so I really don't >> >> >> feel I want to fix the majority of our drivers. I just think such >> >> >> paths are not excercised enough to be seen in practice often or the >> >> >> bugs just get unnoticed. >> >> > >> >> > Eh, many maps for static allocations were already NULL and have been for a >> >> > long time. This is nothign new. Plus, the diff you posted has a bug >> >> > regardless of explicitly destroying a map created by bus_dmamem_alloc(). >> >> >> >> Did you notice that I *removed* the destroy not *added*? >> > >> > Yes, my point was that that bug in the original code you are fixing was there >> > regardless of Scott's change. >> >> And when I did say something different? >> I don't understand what's the point of your messages, besides showing >> that you didn't read correctly my patch. > > I read yours correctly but worded mine poorly. My point is that Scott's > change does not introduce anything new. We've had NULL maps for static > allocations for many, many years. It's only been recently that we've > had more maps not be NULL for this. However, even if you discounted > the whole NULL vs non-NULL maps thing, the driver in question that you > are fixing was broken regardless. That is, due to the extra > bus_dmamap_destroy() the driver was broken regardless of whether the map > was NULL or non-NULL. To be honest, pre-266775 the kernel would actually panic for this specific driver, because we were going to free memory that was never allocated (by having a valid mapping but an invalid dma memory pointer). That was prompted to look at the dma_alloc_*() bits of drivers. We need to make a real sweep at drivers on these bits. > > TL;DR: > > - Scott's change did not introduce any new behavior so we don't need to > worry about a spate of new bugs uncovered in drivers because of it. Not entirely true. For ATA it was before a panic and now it is a silent memory leak. For other drivers it may be the opposite. I could just find this one becaue I got bitten by it. Attilio -- Peace can only be achieved by understanding - A. Einstein
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