Re: [PATCH v2 06/16] mm: add mmap_action_simple_ioremap()

Suren Baghdasaryan
2026-03-17T04:14:49Z

On Mon, Mar 16, 2026 at 2:13 PM Lorenzo Stoakes (Oracle) <ljs@kernel.org> wrote:
>
> Currently drivers use vm_iomap_memory() as a simple helper function for
> I/O remapping memory over a range starting at a specified physical address
> over a specified length.
>
> In order to utilise this from mmap_prepare, separate out the core logic
> into __simple_ioremap_prep(), update vm_iomap_memory() to use it, and add
> simple_ioremap_prepare() to do the same with a VMA descriptor object.
>
> We also add MMAP_SIMPLE_IO_REMAP and relevant fields to the struct
> mmap_action type to permit this operation also.
>
> We use mmap_action_ioremap() to set up the actual I/O remap operation once
> we have checked and figured out the parameters, which makes
> simple_ioremap_prepare() easy to implement.
>
> We then add mmap_action_simple_ioremap() to allow drivers to make use of
> this mode.
>
> We update the mmap_prepare documentation to describe this mode.
>
> Finally, we update the VMA tests to reflect this change.
>
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>

A couple of nits, but otherwise LGTM.

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
> Documentation/filesystems/mmap_prepare.rst | 3 +
> include/linux/mm.h | 24 +++++-
> include/linux/mm_types.h | 6 +-
> mm/internal.h | 2 +
> mm/memory.c | 87 +++++++++++++++-------
> mm/util.c | 12 +++
> tools/testing/vma/include/dup.h | 6 +-
> 7 files changed, 112 insertions(+), 28 deletions(-)
>
> diff --git a/Documentation/filesystems/mmap_prepare.rst b/Documentation/filesystems/mmap_prepare.rst
> index 20db474915da..be76ae475b9c 100644
> --- a/Documentation/filesystems/mmap_prepare.rst
> +++ b/Documentation/filesystems/mmap_prepare.rst
> @@ -153,5 +153,8 @@ pointer. These are:
> * mmap_action_ioremap_full() - Same as mmap_action_ioremap(), only remaps
> the entire mapping from ``start_pfn`` onward.
>
> +* mmap_action_simple_ioremap() - Sets up an I/O remap from a specified
> + physical address and over a specified length.
> +
> **NOTE:** The ``action`` field should never normally be manipulated directly,
> rather you ought to use one of these helpers.
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ad1b8c3c0cfd..df8fa6e6402b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4337,11 +4337,33 @@ static inline void mmap_action_ioremap(struct vm_area_desc *desc,
> * @start_pfn: The first PFN in the range to remap.
> */
> static inline void mmap_action_ioremap_full(struct vm_area_desc *desc,
> - unsigned long start_pfn)
> + unsigned long start_pfn)
> {
> mmap_action_ioremap(desc, desc->start, start_pfn, vma_desc_size(desc));
> }
>
> +/**
> + * mmap_action_simple_ioremap - helper for mmap_prepare hook to specify that the
> + * physical range in [start_phys_addr, start_phys_addr + size) should be I/O
> + * remapped.
> + * @desc: The VMA descriptor for the VMA requiring remap.
> + * @start_phys_addr: Start of the physical memory to be mapped.
> + * @size: Size of the area to map.
> + *
> + * NOTE: Some drivers might want to tweak desc->page_prot for purposes of
> + * write-combine or similar.
> + */
> +static inline void mmap_action_simple_ioremap(struct vm_area_desc *desc,
> + phys_addr_t start_phys_addr,
> + unsigned long size)
> +{
> + struct mmap_action *action = &desc->action;
> +
> + action->simple_ioremap.start_phys_addr = start_phys_addr;
> + action->simple_ioremap.size = size;
> + action->type = MMAP_SIMPLE_IO_REMAP;
> +}
> +
> int mmap_action_prepare(struct vm_area_desc *desc);
> int mmap_action_complete(struct vm_area_struct *vma,
> struct mmap_action *action);
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 4a229cc0a06b..50685cf29792 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -814,6 +814,7 @@ enum mmap_action_type {
> MMAP_NOTHING, /* Mapping is complete, no further action. */
> MMAP_REMAP_PFN, /* Remap PFN range. */
> MMAP_IO_REMAP_PFN, /* I/O remap PFN range. */
> + MMAP_SIMPLE_IO_REMAP, /* I/O remap with guardrails. */
> };
>
> /*
> @@ -822,13 +823,16 @@ enum mmap_action_type {
> */
> struct mmap_action {
> union {
> - /* Remap range. */
> struct {
> unsigned long start;
> unsigned long start_pfn;
> unsigned long size;
> pgprot_t pgprot;
> } remap;
> + struct {
> + phys_addr_t start_phys_addr;
> + unsigned long size;
> + } simple_ioremap;
> };
> enum mmap_action_type type;
>
> diff --git a/mm/internal.h b/mm/internal.h
> index f5774892071e..0eaca2f0eb6a 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1804,6 +1804,8 @@ int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm);
> int remap_pfn_range_prepare(struct vm_area_desc *desc);
> int remap_pfn_range_complete(struct vm_area_struct *vma,
> struct mmap_action *action);
> +int simple_ioremap_prepare(struct vm_area_desc *desc);
> +/* No simple_ioremap_complete, is ultimately handled by remap complete. */
>
> static inline int io_remap_pfn_range_prepare(struct vm_area_desc *desc)
> {
> diff --git a/mm/memory.c b/mm/memory.c
> index 9dec67a18116..f3f4046aee97 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3170,6 +3170,59 @@ int remap_pfn_range_complete(struct vm_area_struct *vma,
> return do_remap_pfn_range(vma, start, pfn, size, prot);
> }
>
> +static int __simple_ioremap_prep(unsigned long vm_start, unsigned long vm_end,

nit: vm_start and vm_end are used only to calculate vm_len. You could
reduce the number of arguments by just passing vm_len.

> + pgoff_t vm_pgoff, phys_addr_t start_phys,
> + unsigned long size, unsigned long *pfnp)
> +{
> + const unsigned long vm_len = vm_end - vm_start;
> + unsigned long pfn, pages;
> +
> + /* Check that the physical memory area passed in looks valid */
> + if (start_phys + size < start_phys)
> + return -EINVAL;
> + /*
> + * You *really* shouldn't map things that aren't page-aligned,
> + * but we've historically allowed it because IO memory might
> + * just have smaller alignment.
> + */
> + size += start_phys & ~PAGE_MASK;
> + pfn = start_phys >> PAGE_SHIFT;
> + pages = (size + ~PAGE_MASK) >> PAGE_SHIFT;
> + if (pfn + pages < pfn)
> + return -EINVAL;
> +
> + /* We start the mapping 'vm_pgoff' pages into the area */
> + if (vm_pgoff > pages)
> + return -EINVAL;
> + pfn += vm_pgoff;
> + pages -= vm_pgoff;
> +
> + /* Can we fit all of the mapping? */
> + if ((vm_len >> PAGE_SHIFT) > pages)
> + return -EINVAL;
> +
> + *pfnp = pfn;
> + return 0;
> +}
> +
> +int simple_ioremap_prepare(struct vm_area_desc *desc)
> +{
> + struct mmap_action *action = &desc->action;
> + const phys_addr_t start = action->simple_ioremap.start_phys_addr;
> + const unsigned long size = action->simple_ioremap.size;
> + unsigned long pfn;
> + int err;
> +
> + err = __simple_ioremap_prep(desc->start, desc->end, desc->pgoff,
> + start, size, &pfn);
> + if (err)
> + return err;
> +
> + /* The I/O remap logic does the heavy lifting. */
> + mmap_action_ioremap(desc, desc->start, pfn, vma_desc_size(desc));

nit: Looks like a perfect opportunity to use mmap_action_ioremap_full() here.

> + return mmap_action_prepare(desc);

Ok, so IIUC this uses recursion:
mmap_action_prepare(MMAP_SIMPLE_IO_REMAP) -> simple_ioremap_prepare()
-> mmap_action_prepare(MMAP_IO_REMAP_PFN).

> +}
> +
> /**
> * vm_iomap_memory - remap memory to userspace
> * @vma: user vma to map to
> @@ -3187,32 +3240,16 @@ int remap_pfn_range_complete(struct vm_area_struct *vma,
> */
> int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned long len)
> {
> - unsigned long vm_len, pfn, pages;
> -
> - /* Check that the physical memory area passed in looks valid */
> - if (start + len < start)
> - return -EINVAL;
> - /*
> - * You *really* shouldn't map things that aren't page-aligned,
> - * but we've historically allowed it because IO memory might
> - * just have smaller alignment.
> - */
> - len += start & ~PAGE_MASK;
> - pfn = start >> PAGE_SHIFT;
> - pages = (len + ~PAGE_MASK) >> PAGE_SHIFT;
> - if (pfn + pages < pfn)
> - return -EINVAL;
> -
> - /* We start the mapping 'vm_pgoff' pages into the area */
> - if (vma->vm_pgoff > pages)
> - return -EINVAL;
> - pfn += vma->vm_pgoff;
> - pages -= vma->vm_pgoff;
> + const unsigned long vm_start = vma->vm_start;
> + const unsigned long vm_end = vma->vm_end;
> + const unsigned long vm_len = vm_end - vm_start;
> + unsigned long pfn;
> + int err;
>
> - /* Can we fit all of the mapping? */
> - vm_len = vma->vm_end - vma->vm_start;
> - if (vm_len >> PAGE_SHIFT > pages)
> - return -EINVAL;
> + err = __simple_ioremap_prep(vm_start, vm_end, vma->vm_pgoff, start,
> + len, &pfn);
> + if (err)
> + return err;
>
> /* Ok, let it rip */
> return io_remap_pfn_range(vma, vma->vm_start, pfn, vm_len, vma->vm_page_prot);
> diff --git a/mm/util.c b/mm/util.c
> index cdfba09e50d7..aa92e471afe1 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1390,6 +1390,8 @@ int mmap_action_prepare(struct vm_area_desc *desc)
> return remap_pfn_range_prepare(desc);
> case MMAP_IO_REMAP_PFN:
> return io_remap_pfn_range_prepare(desc);
> + case MMAP_SIMPLE_IO_REMAP:
> + return simple_ioremap_prepare(desc);
> }
>
> WARN_ON_ONCE(1);
> @@ -1421,6 +1423,14 @@ int mmap_action_complete(struct vm_area_struct *vma,
> case MMAP_IO_REMAP_PFN:
> err = io_remap_pfn_range_complete(vma, action);
> break;
> + case MMAP_SIMPLE_IO_REMAP:
> + /*
> + * The simple I/O remap should have been delegated to an I/O
> + * remap.
> + */
> + WARN_ON_ONCE(1);
> + err = -EINVAL;
> + break;
> }
>
> return mmap_action_finish(vma, action, err);
> @@ -1434,6 +1444,7 @@ int mmap_action_prepare(struct vm_area_desc *desc)
> break;
> case MMAP_REMAP_PFN:
> case MMAP_IO_REMAP_PFN:
> + case MMAP_SIMPLE_IO_REMAP:
> WARN_ON_ONCE(1); /* nommu cannot handle these. */
> break;
> }
> @@ -1452,6 +1463,7 @@ int mmap_action_complete(struct vm_area_struct *vma,
> break;
> case MMAP_REMAP_PFN:
> case MMAP_IO_REMAP_PFN:
> + case MMAP_SIMPLE_IO_REMAP:
> WARN_ON_ONCE(1); /* nommu cannot handle this. */
>
> err = -EINVAL;
> diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
> index 4570ec77f153..114daaef4f73 100644
> --- a/tools/testing/vma/include/dup.h
> +++ b/tools/testing/vma/include/dup.h
> @@ -453,6 +453,7 @@ enum mmap_action_type {
> MMAP_NOTHING, /* Mapping is complete, no further action. */
> MMAP_REMAP_PFN, /* Remap PFN range. */
> MMAP_IO_REMAP_PFN, /* I/O remap PFN range. */
> + MMAP_SIMPLE_IO_REMAP, /* I/O remap with guardrails. */
> };
>
> /*
> @@ -461,13 +462,16 @@ enum mmap_action_type {
> */
> struct mmap_action {
> union {
> - /* Remap range. */
> struct {
> unsigned long start;
> unsigned long start_pfn;
> unsigned long size;
> pgprot_t pgprot;
> } remap;
> + struct {
> + phys_addr_t start;
> + unsigned long len;
> + } simple_ioremap;
> };
> enum mmap_action_type type;
>
> --
> 2.53.0
>