On 10/2/26 15:59, Lorenzo Stoakes (ARM) wrote:
> On Fri, Oct 02, 2026 at 03:11:27PM +0200, David Hildenbrand (Arm) wrote:
>> I'd be very happy if we could find a clear way to describe "this is what we
>> call
>> user memory: anon+pagecache", if that could come handy in such a context.
>
> Rather than arguing about it, how about shifting things to 'what backs the
> mapping' and something like:
>
> /* big long kdoc... */
> static inline bool vma_flags_is_mm_backed(const vma_flags_t *flags)
> {
> /* hugetlb is a fixed mapping, but the mm owns it entirely. */
> if (vma_flags_is_hugetlb(flags))
> return true;
>
> /*
> * Kernel-owned mappings are populated by their owner, and fixed
> mappings
> * have a layout the mm cannot assume ordinary fault semantics over.
> */
> if (vma_flags_is_kernel_owned(flags) ||
> vma_flags_is_fixed_mapping(flags))
> return false;
>
> /* The mm has promised it may discard droppable memory at any time. */
> return !vma_flags_test_single_mask(flags, VMA_DROPPABLE);
> }
In my other mail I was wondering whether we could use vma_is_mm_managed() to
express !vma_is_kernel_owned(). Maybe vma_is_mm_backed() could fit into that
picture.
Reading above I am still confused why something that expresses "backed" talks
about "fixed mappings and ownership", sorry :(
It's late here, and this is a hard nut to crack.
What we have here is:
1) Is this an ordinary MM-managed VMA. So far so good, I understand that. (with
a twist what I just learned about things that map random allocated pages
through -> fault, but so far so good)
2) Is this MM "fixed" that makes expand/merge tricky, except if it's hugetlb
where we can still handle it.
I think VM_DONTEXPAND is also rather misnamed :( I assume it really means, using
sel_mmap_policy_ops() as an example: the MM manages the things that are getting
mapped in here (->fault), but the pages are not really pagecache/anon, but some
other shit. So we cannot expand the mapping.
Gah, so complicated.
3) Can the MM drop the pages at any time.
I still think that 3) does not quite fit into that picture. Using something like
"vma_is_mm_backed" to say "this is mm-managed, but all things in there come from
core-mm and not some other random shit people punch into ->fault" would work
for me.
(not sure if there is still some other way how someone could get random pages
into a VMA ... or how to catch someone not setting the DONTEXPAND flag)
God, this is all such a confusing mess with so many ways of getting stuff messed
up by other parts of the system. Thanks for working on cleaning that up.
--
Cheers,
David
