Re: [Intel-gfx] [PATCH 2/4] mm: add a io_mapping_map_user helper

[Lucas De Marchi <lucas.demarchi@intel.com>]

On Fri, Mar 26, 2021 at 06:55:03AM +0100, Christoph Hellwig wrote:
>Add a helper that calls remap_pfn_range for an struct io_mapping, relying
>on the pgprot pre-validation done when creating the mapping instead of
>doing it at runtime.
>
>Signed-off-by: Christoph Hellwig <hch@lst.de>
>---
> include/linux/io-mapping.h |  3 +++
> mm/Kconfig                 |  3 +++
> mm/Makefile                |  1 +
> mm/io-mapping.c            | 29 +++++++++++++++++++++++++++++
> 4 files changed, 36 insertions(+)
> create mode 100644 mm/io-mapping.c
>
>diff --git a/include/linux/io-mapping.h b/include/linux/io-mapping.h
>index c093e81310a9b3..e9743cfd858527 100644
>--- a/include/linux/io-mapping.h
>+++ b/include/linux/io-mapping.h
>@@ -220,3 +220,6 @@ io_mapping_free(struct io_mapping *iomap)
> }
>
> #endif /* _LINUX_IO_MAPPING_H */
>+
>+int io_mapping_map_user(struct io_mapping *iomap, struct vm_area_struct *vma,
>+		unsigned long addr, unsigned long pfn, unsigned long size);

I'm not sure what exactly brought me to check this, but while debugging
I noticed this outside the header guard. But then after some more checks I
saw nothing actually selects CONFIG_IO_MAPPING because commit using
it was reverted in commit 0e4fe0c9f2f9 ("Revert "i915: use io_mapping_map_user"")

Is this something we want to re-attempt moving to mm/ ?

thanks
Lucas De Marchi

>diff --git a/mm/Kconfig b/mm/Kconfig
>index 24c045b24b9506..6b0f2cfbfac0f3 100644
>--- a/mm/Kconfig
>+++ b/mm/Kconfig
>@@ -872,4 +872,7 @@ config MAPPING_DIRTY_HELPERS
> config KMAP_LOCAL
> 	bool
>
>+# struct io_mapping based helper.  Selected by drivers that need them
>+config IO_MAPPING
>+	bool
> endmenu
>diff --git a/mm/Makefile b/mm/Makefile
>index 72227b24a61688..c0135e385984bb 100644
>--- a/mm/Makefile
>+++ b/mm/Makefile
>@@ -120,3 +120,4 @@ obj-$(CONFIG_MEMFD_CREATE) += memfd.o
> obj-$(CONFIG_MAPPING_DIRTY_HELPERS) += mapping_dirty_helpers.o
> obj-$(CONFIG_PTDUMP_CORE) += ptdump.o
> obj-$(CONFIG_PAGE_REPORTING) += page_reporting.o
>+obj-$(CONFIG_IO_MAPPING) += io-mapping.o
>diff --git a/mm/io-mapping.c b/mm/io-mapping.c
>new file mode 100644
>index 00000000000000..01b3627999304e
>--- /dev/null
>+++ b/mm/io-mapping.c
>@@ -0,0 +1,29 @@
>+// SPDX-License-Identifier: GPL-2.0-only
>+
>+#include <linux/mm.h>
>+#include <linux/io-mapping.h>
>+
>+/**
>+ * io_mapping_map_user - remap an I/O mapping to userspace
>+ * @iomap: the source io_mapping
>+ * @vma: user vma to map to
>+ * @addr: target user address to start at
>+ * @pfn: physical address of kernel memory
>+ * @size: size of map area
>+ *
>+ *  Note: this is only safe if the mm semaphore is held when called.
>+ */
>+int io_mapping_map_user(struct io_mapping *iomap, struct vm_area_struct *vma,
>+		unsigned long addr, unsigned long pfn, unsigned long size)
>+{
>+	vm_flags_t expected_flags = VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
>+
>+	if (WARN_ON_ONCE((vma->vm_flags & expected_flags) != expected_flags))
>+		return -EINVAL;
>+
>+	/* We rely on prevalidation of the io-mapping to skip track_pfn(). */
>+	return remap_pfn_range_notrack(vma, addr, pfn, size,
>+		__pgprot((pgprot_val(iomap->prot) & _PAGE_CACHE_MASK) |
>+			 (pgprot_val(vma->vm_page_prot) & ~_PAGE_CACHE_MASK)));
>+}
>+EXPORT_SYMBOL_GPL(io_mapping_map_user);
>-- 
>2.30.1
>
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