From: Peter Zijlstra > Sent: 13 May 2020 14:36 > > On Wed, May 13, 2020 at 10:10:03AM +0000, David Laight wrote: > > From: Geert Uytterhoeven > > > Sent: 13 May 2020 10:49 > > ... > > > > I don't want to apply this to older kernels as it could cause extra > > > > memory usage for no good reason. I have no idea why a non ARC system > > > > would want it :( > > > > > > I think the reference to ARC is a red herring. > > > The real issue is that buffers used for DMA may not have the required > > > alignment, which is not limited to ARC systems. > > > > > > Note that I'm also not super happy with the extra memory usage. > > > But devm_*() conveniences come with their penalties... > > > > Interesting thought. > > Could the devm 'header' be put right at the end of the kmalloc() > > buffer? > > https://lkml.kernel.org/r/20191220140655.GN2827@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx All the way around the loop..... ISTR there have also been issues with one of the kmalloc() implementations adding a header to the memory block. Didn't it generate 4n+2 aligned buffers on m68k - breaking code that tried to use the two lower bits of an address. If one of the kmalloc()s behaves like that it ought to be possible for devm_alloc() to use the spare space in the same cache line?? David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
