On Tue, Jun 22, 2021 at 02:21:27PM +0000, David Laight wrote: > From: Alan Stern > > Sent: 22 June 2021 14:29 > ... > > > Thought... > > > > > > Is kmalloc(1, GFP_KERNEL) guaranteed to return a pointer into > > > a cache line that will not be accessed by any other code? > > > (This is slightly weaker than requiring a cache-line aligned > > > pointer - but very similar.) > > > > As I understand it, on architectures that do not have cache-coherent > > I/O, kmalloc is guaranteed to return a buffer that is > > cacheline-aligned and whose length is a multiple of the cacheline > > size. > > > > Now, whether that buffer ends up being accessed by any other code > > depends on what your driver does with the pointer it gets from > > kmalloc. :-) > > Thanks for the clarification. > > Most of the small allocates in the usb stack are for transmits > where it is only necessary to ensure a cache write-back. > > I know there has been some confusion because one of the > allocators can add a small header to every allocation. > This can lead to unexpectedly inadequately aligned pointers. > If it is updated when the preceding block is freed (as some > user-space mallocs do) then it would need to be in a > completely separate cache line. If you really want to find out what the true story is, you should ask on the linux-mm mailing list. The rest of us are not experts on this stuff. Alan Stern
