On Mon, May 19, 2025 at 08:14:17PM +0100, Lorenzo Stoakes wrote:
> On Mon, May 19, 2025 at 08:59:32PM +0200, David Hildenbrand wrote:
> > > >
> > > > I am not 100% sure why we bail out on special mappings: all we have to do is
> > > > reliably identify anon pages, and we should be able to do that.
> > >
> > > But they map e.g. kernel memory (at least for VM_PFNMAP, purely and by
> > > implication really VM_IO), it wouldn't make sense for KSM to be asked to
> > > try to merge these right?
> > >
> > > And of course no underlying struct page to pin, no reference counting
> > > either, so I think you'd end up in trouble potentially here wouldn't you?
> > > And how would the CoW work?
> >
> > KSM only operates on anonymous pages. It cannot de-duplicate anything else.
> > (therefore, only MAP_PRIVATE applies)
>
> Yes I had this realisation see my reply to your reply :)
>
> I mean is MAP_PRIVATE of a VM_PFNMAP really that common?...
>
> >
> > Anything else (no struct page, not a CoW anon folio in such a mapping) is
> > skipped.
> >
> > Take a look at scan_get_next_rmap_item() where we do
> >
> > folio = folio_walk_start(&fw, vma, ksm_scan.address, 0);
> > if (folio) {
> > if (!folio_is_zone_device(folio) &&
> > folio_test_anon(folio)) {
> > folio_get(folio);
> > tmp_page = fw.page;
> > }
> > folio_walk_end(&fw, vma)
> > }
> >
> >
> > Before I changed that code, we were using GUP. And GUP just always refuses
> > VM_IO|VM_PFNMAP because it cannot handle it properly.
>
> OK so it boils down to doing KSM _on the already CoW'd_ MAP_PRIVATE mapping?
>
> But hang on, how do we discover this? vm_normal_page() will screw this up right?
> As VM_SPECIAL will be set...
>
> OK now I'm not sure I understand how MAP_PRIVATE-mapped VM_PFNMAP mappings work
> :)))
Oh wait hang on...
So here, MAP_PRIVATE, CoW'd would clear pte_special() presumably:
if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
if (likely(!pte_special(pte)))
goto check_pfn;
...
And in non-PTE special flag case:
...
if (!is_cow_mapping(vma->vm_flags))
return NULL;
...
And of course, this will be a CoW mapping...
So actually we should be fine. Got it.
>
> >
> > > >
> > > > So, assuming we could remove the VM_PFNMAP | VM_IO | VM_DONTEXPAND |
> > > > VM_MIXEDMAP constraint from vma_ksm_compatible(), could we simplify?
> > >
> > > Well I question removing this constraint for above reasons.
> > >
> > > At any rate, even if we _could_ this feels like a bigger change that we
> > > should come later.
> >
> > "bigger" -- it might just be removing these 4 flags from the check ;)
> >
> > I'll dig a bit more.
>
> Right, but doing so would be out of scope here don't you think?
>
> I'd rather not delay fixing this bug on this basis ideally, esp. as easy to
> adjust later.
>
> I suggest we put this in as-is (or close to it anyway) and then if we remove the
> flags we can change this...
>
> As I said in other reply, .mmap() means the driver can do literally anything
> they want (which is _hateful_), so we'd really want to have some confidence they
> didn't do something so crazy, unless we were happy to just let that possibly
> explode.
>
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >
