On 12.02.24 21:38, Ryan Roberts wrote:
[...]
+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
