On Sun, 27 Mar 2022 17:30:01 -0700 Linus Torvalds torvalds@linux-foundation.org wrote:
On Sun, Mar 27, 2022 at 4:52 PM Halil Pasic pasic@linux.ibm.com wrote:
I have no intention of pursuing this. When fixing the information leak, I happened to realize, that a somewhat similar situation can emerge when mappings are reused. It seemed like an easy fix, so I asked the swiotlb maintainers, and they agreed. It ain't my field of expertise, and the drivers I'm interested in don't need this functionality.
Ok.
That said, I think you are putting yourself down when you said in an earlier email that you aren't veryt knowledgeable in this area.
I think the fact that you *did* think of this other similar situation is actually very interesting, and it's something people probably _haven't_ been thinking about.
Thank you!
So I think your first commit fixes the straightforward and common case where you do that "map / partial dma / unmap" case.
And that straightforward case is probably all that the disk IO case ever really triggers, which is presumably why those "drivers I'm interested in don't need this functionality" don't need anything else?
I agree.
And yes, your second commit didn't work, but hey, whatever. The whole "multiple operations on the same double buffering allocation" situation is something I don't think people have necessarily thought about enough.
And by that I don't mean you. I mean very much the whole history of our dma mapping code.
I agree. We are in the process of catching up! :) My idea was to aid a process, as a relatively naive pair of eyes: somebody didn't read any data sheets describing non-cache-coherent DMA, and never programmed a DMA. It is a fairly common problem, that for the very knowledgeable certain things seem obvious, self-explanatory or trivial, but for the less knowledgeable the are not. And knowledge can create bias.
I then get opinionated and probably too forceful, but please don't take it as being about you - it's about just my frustration with that code - and if it comes off too negative then please accept my apologies.
I have to admit, I did feel a little uncomfortable, and I did look for an exit strategy. I do believe, that people in your position do have to occasionally get forceful, and even abrasive to maintain efficiency. I try to not ignore the social aspect of things, but I do get carried away occasionally.
Your last especially paragraph is very encouraging and welcome. Thank you!
Regards, Halil
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