On 12.02.24 21:38, Ryan Roberts wrote:

[...]

quoted

quoted

quoted

quoted

+static inline bool mm_is_user(struct mm_struct *mm)
+{
+	/*
+	 * Don't attempt to apply the contig bit to kernel mappings, because
+	 * dynamically adding/removing the contig bit can cause page faults.
+	 * These racing faults are ok for user space, since they get serialized
+	 * on the PTL. But kernel mappings can't tolerate faults.
+	 */
+	return mm != &init_mm;
+}

We also have the efi_mm as a non-user mm, though I don't think we manipulate
that while it is live, and I'm not sure if that needs any special handling.

Well we never need this function in the hot (order-0 folio) path, so I think I
could add a check for efi_mm here with performance implication. It's probably
safest to explicitly exclude it? What do you think?

Oops: This should have read "I think I could add a check for efi_mm here
*without* performance implication"

It turns out that efi_mm is only defined when CONFIG_EFI is enabled. I can do this:

return mm != &init_mm && (!IS_ENABLED(CONFIG_EFI) || mm != &efi_mm);

Please use all the lines you need ;)

if (IS_ENABLED(CONFIG_EFI) && unlikely(mm == &efi_mm))
	return false;
return mm != &init_mm;

Is that acceptable? This is my preference, but nothing else outside of efi
references this symbol currently.

We could also mark MMs in some way to be special.

return mm->is_user;

Then it's easy to extend.

-- 
Cheers,

David / dhildenb