On 11/7/24 16:10, Matthew Wilcox wrote:
On Thu, Nov 07, 2024 at 02:24:20PM +0530, Shivank Garg wrote:The folio allocation path from guest_memfd typically looks like this... kvm_gmem_get_folio filemap_grab_folio __filemap_get_folio filemap_alloc_folio __folio_alloc_node_noprof -> goes to the buddy allocator Hence, I am trying to have a version of filemap_alloc_folio() that takes an mpol.It only takes that path if cpuset_do_page_mem_spread() is true. Is the real problem that you're trying to solve that cpusets are being used incorrectly?
If it's false it's not very different, it goes to alloc_pages_noprof(). Then it respects the process's policy, but the policy is not customizable without mucking with state that is global to the process.
Taking a step back: the problem is that a VM can be configured to have multiple guest-side NUMA nodes, each of which will pick memory from the right NUMA node in the host. Without a per-file operation it's not possible to do this on guest_memfd. The discussion was whether to use ioctl() or a new system call. The discussion ended with the idea of posting a *proposal* asking for *comments* as to whether the system call would be useful in general beyond KVM.
Commenting on the system call itself I am not sure I like the file_operations entry, though I understand that it's the simplest way to implement this in an RFC series. It's a bit surprising that fbind() is a total no-op for everything except KVM's guest_memfd.
Maybe whatever you pass to fbind() could be stored in the struct file *, and used as the default when creating VMAs; as if every mmap() was followed by an mbind(), except that it also does the right thing with MAP_POPULATE for example. Or maybe that's a horrible idea?
Adding linux-api to get input; original thread is at https://lore.kernel.org/kvm/20241105164549.154700-1-shivankg@xxxxxxx/. Paolo
Backing up, it seems like you want to make a change to the page cache, you've had a long discussion with people who aren't the page cache maintainer, and you all understand the pros and cons of everything, and here you are dumping a solution on me without talking to me, even though I was at Plumbers, you didn't find me to tell me I needed to go to your talk. So you haven't explained a damned thing to me, and I'm annoyed at you. Do better. Starting with your cover letter.
