Re: [PATCH v3 2/8] binder: concurrent page installation

From: David Hildenbrand
Date: Tue Nov 12 2024 - 09:59:13 EST


On 12.11.24 15:43, Carlos Llamas wrote:
On Tue, Nov 12, 2024 at 12:10:20PM +0100, David Hildenbrand wrote:
On 08.11.24 20:10, Carlos Llamas wrote:
+ ret = vm_insert_page(vma, addr, page);
+ switch (ret) {
+ case -EBUSY:
+ /*
+ * EBUSY is ok. Someone installed the pte first but the
+ * lru_page->page_ptr has not been updated yet. Discard
+ * our page and look up the one already installed.
+ */
+ ret = 0;
+ __free_page(page);
+ npages = get_user_pages_remote(alloc->mm, addr, 1, 0, &page, NULL);

This will trigger a page fault if we don't find what we expect (are races
with e.g., MADV_DONTNEED possible?), is that really desired or not a
problem?

This is fine. As of now, binder blocks its page faults:

static vm_fault_t binder_vm_fault(struct vm_fault *vmf)
{
return VM_FAULT_SIGBUS;
}

If we race with something like MADV_DONTNEED then we would just
propagate the -EFAULT error. I could add FOLL_NOFAULT to the gup remote
call to make it evident we don't care though.

Might make it clearer ... or at least adding a comment how this is supposed to work. :)

--
Cheers,

David / dhildenb