On Wed, Apr 13, 2022 at 09:53:24AM -1000, Linus Torvalds wrote: > On Tue, Apr 12, 2022 at 10:47 PM Catalin Marinas > <catalin.marinas@xxxxxxx> wrote: > > > > I agree. There is also an implicit expectation that the DMA API works on > > kmalloc'ed buffers and that's what ARCH_DMA_MINALIGN is for (and the > > dynamic arch_kmalloc_minalign() in this series). But the key point is > > that the driver doesn't need to know the CPU cache topology, coherency, > > the DMA API and kmalloc() take care of these. > > Honestly, I think it would probably be worth discussing the "kmalloc > DMA alignment" issues. > > 99.9% of kmalloc users don't want to do DMA. > > And there's actually a fair amount of small kmalloc for random stuff. > Right now on my laptop, I have > > kmalloc-8 16907 18432 8 512 1 : ... > > according to slabinfo, so almost 17 _thousand_ allocations of 8 bytes. > > It's all kinds of sad if those allocations need to be 64 bytes in size > just because of some silly DMA alignment issue, when none of them want > it. > > Yeah, yeah, wasting a megabyte of memory is "just a megabyte" these > days. Which is crazy. It's literally memory that could have been used > for something much more useful than just pure and utter waste. > > I think we could and should just say "people who actually require DMA > accesses should say so at kmalloc time". We literally have that > GFP_DMA and ZOME_DMA for various historical reasons, so we've been > able to do that before. > > No, that historical GFP_DMA isn't what arm64 wants - it's the old > crazy "legacy 16MB DMA" thing that ISA DMA used to have. > > But the basic issue was true then, and is true now - DMA allocations > are fairly special, and should not be that hard to just mark as such. "fairly special" == "all USB transactions", so it will take a lot of auditing here. I think also many SPI controllers require this and maybe I2C? Perhaps other bus types do as well. So please don't make this change without some way of figuring out just what drivers need to be fixed up, as it's going to be a lot... thanks, greg k-h
