Page Detective will use it to walk the kernel page table. Make this
function accessible from modules, and also while here make
walk_page_range() accessible from modules, so Page Detective could
use it to walk user page tables.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/pagewalk.h |  2 ++
 mm/pagewalk.c            | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index f5eb5a32aeed..ff25374470f0 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -124,6 +124,8 @@ int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
 int walk_page_range_vma(struct vm_area_struct *vma, unsigned long start,
 			unsigned long end, const struct mm_walk_ops *ops,
 			void *private);
+int walk_page_range_kernel(unsigned long start, unsigned long end,
+			   const struct mm_walk_ops *ops, void *private);
 int walk_page_vma(struct vm_area_struct *vma, const struct mm_walk_ops *ops,
 		void *private);
 int walk_page_mapping(struct address_space *mapping, pgoff_t first_index,
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 5f9f01532e67..050790aeb15f 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -478,6 +478,7 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
 	} while (start = next, start < end);
 	return err;
 }
+EXPORT_SYMBOL_GPL(walk_page_range);
 
 /**
  * walk_page_range_novma - walk a range of pagetables not backed by a vma
@@ -541,6 +542,37 @@ int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
 	return walk_pgd_range(start, end, &walk);
 }
 
+/**
+ * walk_page_range_kernel - walk a range of pagetables of kernel/init_mm
+ * @start:	start address of the virtual address range
+ * @end:	end address of the virtual address range
+ * @ops:	operation to call during the walk
+ * @private:	private data for callbacks' usage
+ *
+ * Similar to walk_page_range_novma() but specifically walks init_mm.pgd table.
+ *
+ * Note: This function takes two looks: get_online_mems(), and mmap_read, this
+ * is to prevent kernel page tables from being freed while walking.
+ */
+int walk_page_range_kernel(unsigned long start, unsigned long end,
+			   const struct mm_walk_ops *ops, void *private)
+{
+		get_online_mems();
+		if (mmap_read_lock_killable(&init_mm)) {
+			put_online_mems();
+			return -EAGAIN;
+		}
+
+		walk_page_range_novma(&init_mm, start, end, ops,
+				      init_mm.pgd, private);
+
+		mmap_read_unlock(&init_mm);
+		put_online_mems();
+
+		return 0;
+}
+EXPORT_SYMBOL_GPL(walk_page_range_kernel);
+
 int walk_page_range_vma(struct vm_area_struct *vma, unsigned long start,
 			unsigned long end, const struct mm_walk_ops *ops,
 			void *private)
-- 
2.47.0.338.g60cca15819-goog

On Sat, Nov 16, 2024 at 05:59:18PM +0000, Pasha Tatashin wrote:
>  	} while (start = next, start < end);
>  	return err;
>  }
> +EXPORT_SYMBOL_GPL(walk_page_range);

Umm, no.  We really should not expose all these page table detail
to modules.

> +EXPORT_SYMBOL_GPL(walk_page_range_kernel);

Even more so here.

On Mon, Nov 18, 2024 at 1:49 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Sat, Nov 16, 2024 at 05:59:18PM +0000, Pasha Tatashin wrote:
> >       } while (start = next, start < end);
> >       return err;
> >  }
> > +EXPORT_SYMBOL_GPL(walk_page_range);
>
> Umm, no.  We really should not expose all these page table detail
> to modules.
>
> > +EXPORT_SYMBOL_GPL(walk_page_range_kernel);
>
> Even more so here.

I will remove these exports in the next version, as I am going to
convert Page Detective to be part of core mm instead of misc device.

Thanks,
Pasha

On Sun, Nov 17, 2024 at 10:49:06PM -0800, Christoph Hellwig wrote:
> On Sat, Nov 16, 2024 at 05:59:18PM +0000, Pasha Tatashin wrote:
> >  	} while (start = next, start < end);
> >  	return err;
> >  }
> > +EXPORT_SYMBOL_GPL(walk_page_range);
>
> Umm, no.  We really should not expose all these page table detail
> to modules.
>
> > +EXPORT_SYMBOL_GPL(walk_page_range_kernel);
>
> Even more so here.
>

Very much agree. You basically then have the ability for any (GPL) driver to
come in and modify page tables at will which is VERY MUCH not a good idea.

The rules around page table manipulation are very subtle and constantly
changing, this is not something for anything outside of mm to be fiddling with.

Again, I find it bizarre we're exporting mm internal implementation details to a
driver to do stuff with rather than adding functionality to mm.