feat(mapper/remap_with_full): conditional remap

This commit modifies the transform function (`f`) argument of
`remap_with_full` to return an `Option`. This allows the caller to skip
remaps based on the previous frame and page flags. It can alternatively
be done by first translating the address and then remapping based on
that but that would mean we have to walk the page tables twice :|

Signed-off-by: Anhad Singh <andypython@protonmail.com>

src/page/mapper.rs

@@ -63,14 +63,16 @@ impl<A: Arch, F: FrameAllocator> PageMapper<A, F> {
     pub unsafe fn remap_with_full(
         &mut self,
         virt: VirtualAddress,
-        f: impl FnOnce(PhysicalAddress, PageFlags<A>) -> (PhysicalAddress, PageFlags<A>),
+        f: impl FnOnce(PhysicalAddress, PageFlags<A>) -> Option<(PhysicalAddress, PageFlags<A>)>,
     ) -> Option<(PageFlags<A>, PhysicalAddress, PageFlush<A>)> {
         unsafe {
             self.visit(virt, |p1, i| {
                 let old_entry = p1.entry(i)?;
                 let old_phys = old_entry.address().ok()?;
                 let old_flags = old_entry.flags();
-                let (new_phys, new_flags) = f(old_phys, old_flags);
+                let Some((new_phys, new_flags)) = f(old_phys, old_flags) else {
+                    return None;
+                };
                 // TODO: Higher-level PageEntry::new interface?
                 let new_entry = PageEntry::new(new_phys.data(), new_flags.data());
                 p1.set_entry(i, new_entry);
@@ -86,7 +88,7 @@ impl<A: Arch, F: FrameAllocator> PageMapper<A, F> {
     ) -> Option<(PageFlags<A>, PhysicalAddress, PageFlush<A>)> {
         unsafe {
             self.remap_with_full(virt, |same_phys, old_flags| {
-                (same_phys, map_flags(old_flags))
+                Some((same_phys, map_flags(old_flags)))
             })
         }
     }