On Wed, Feb 07, 2024 at 09:07:51PM +0000, Lorenzo Stoakes wrote: > On Tue, Feb 06, 2024 at 02:04:28PM -0800, Alexei Starovoitov wrote: > > From: Alexei Starovoitov <ast@xxxxxxxxxx> > > > > The next commit will introduce bpf_arena which is a sparsely populated shared > > memory region between bpf program and user space process. > > It will function similar to vmalloc()/vm_map_ram(): > > - get_vm_area() > > - alloc_pages() > > - vmap_pages_range() > > This tells me absolutely nothing about why it is justified to expose this > internal interface. You need to put more explanation here along the lines > of 'we had no other means of achieving what we needed from vmalloc because > X, Y, Z and are absolutely convinced it poses no risk of breaking anything'. How about this: --- BPF would like to use the vmap API to implement a lazily-populated memory space which can be shared by multiple userspace threads. The vmap API is generally public and has functions to request and release areas of kernel address space, as well as functions to map various types of backing memory into that space. For example, there is the public ioremap_page_range(), which is used to map device memory into addressable kernel space. The new BPF code needs the functionality of vmap_pages_range() in order to incrementally map privately managed arrays of pages into its vmap area. Indeed this function used to be public, but became private when usecases other than vmalloc happened to disappear. Make it public again for the new external user. --- > I mean I see a lot of checks in vmap() that aren't in vmap_pages_range() > for instance. We good to expose that, not only for you but for any other > core kernel users? Those are applicable only to the higher-level vmap/vmalloc usecases: controlling the implied call to get_vm_area; managing the area with vfree(). They're not relevant for mapping privately-managed pages into an existing vm area. It's the same pattern and layer of abstraction as ioremap_pages_range(), which doesn't have any of those checks either.
