Do we have any caller using vm_page_grab_pages(_unlocked)(VM_ALLOC_ZERO)? I did not found any.

I thought what could be a use for such call, and I really do not see how it can be useful. Both vnode and swap objects might have the page content on the volume, so the invalid page we found on the queue cannot be safely zeroed without consulting pager first. In the end, the decision to zero such page belongs to the caller.

In D56432#1292906, @kib wrote:

Do we have any caller using vm_page_grab_pages(_unlocked)(VM_ALLOC_ZERO)? I did not found any.

kern_kexec.c calls vm_page_grab_pages_unlocked(VM_ALLOC_ZERO). It looks like it could just use vm_page_alloc() though.

I thought what could be a use for such call, and I really do not see how it can be useful. Both vnode and swap objects might have the page content on the volume, so the invalid page we found on the queue cannot be safely zeroed without consulting pager first. In the end, the decision to zero such page belongs to the caller.

Effectively, you agree that we can drop the support and disable passing VM_ALLOC_ZERO to vm_page_grab_unlocked?

In D56432#1294556, @kib wrote:

Effectively, you agree that we can drop the support and disable passing VM_ALLOC_ZERO to vm_page_grab_unlocked?

After kern_kexec is fixed, yes. But now I am trying to understand what that code is doing, and failing.

In D56432#1292906, @kib wrote:

I thought what could be a use for such call, and I really do not see how it can be useful. Both vnode and swap objects might have the page content on the volume, so the invalid page we found on the queue cannot be safely zeroed without consulting pager first. In the end, the decision to zero such page belongs to the caller.

For other object types, like OBJT_PHYS in kern_kexec.c, it is still somewhat useful since we do not have a vm_page_alloc_pages(), so pages have to be allocated one-by-one. But yes, also note that vm_page_grab_valid() does not handle VM_ALLOC_ZERO.

In D56432#1294557, @markj wrote:

In D56432#1294556, @kib wrote:

Effectively, you agree that we can drop the support and disable passing VM_ALLOC_ZERO to vm_page_grab_unlocked?

After kern_kexec is fixed, yes. But now I am trying to understand what that code is doing, and failing.

In D56432#1292906, @kib wrote:

I thought what could be a use for such call, and I really do not see how it can be useful. Both vnode and swap objects might have the page content on the volume, so the invalid page we found on the queue cannot be safely zeroed without consulting pager first. In the end, the decision to zero such page belongs to the caller.

For other object types, like OBJT_PHYS in kern_kexec.c, it is still somewhat useful since we do not have a vm_page_alloc_pages(), so pages have to be allocated one-by-one. But yes, also note that vm_page_grab_valid() does not handle VM_ALLOC_ZERO.

Even so, using the unlocked variant is utterly pointless, since there can't possibly be any existing pages in the object.