Re: [PATCH] drm/dma-helpers: Don't change vma flags
On Thu, Nov 24, 2022 at 06:24:52PM +0100, Daniel Vetter wrote:
> On Thu, Nov 24, 2022 at 11:11:21AM +, Robin Murphy wrote:
> > On 2022-11-23 17:28, Daniel Vetter wrote:
> > > This code was added in b65e64f7ccd4 ("drm/cma: Use
> > > dma_mmap_writecombine() to mmap buffer"), but does not explain why
> > > it's needed.
> > >
> > > It should be entirely unnecessary, because remap_pfn_range(), which is
> > > what the various dma_mmap functiosn are built on top of, does already
> > > unconditionally adjust the vma flags:
> >
> > Not all dma_mmap_*() implementations use remap_pfn_range() though. For
> > instance on ARM where one set of DMA ops uses vm_map_pages(), but AFAICS not
> > all the relevant drivers would set VM_MIXEDMAP to prevent reaching the
> > BUG_ON(vma->vm_flags & VM_PFNMAP) in there.
>
> Uh a dma_mmap which does not use VM_PFNMAP has pretty good chances of
> being busted, since that allows get_user_pages on these memory
> allocations. And I'm really not sure that's a bright idea ...
>
> Can you please point me at these dma-ops so that I can try and understand
> what they're trying to do?
I've taken a trip back in time to v3.14, when b65e64f7ccd4 was merged,
and checked the x86, arm and arm64 dma mapping implementations. They all
call remap_pfn_range() except for arm_iommu_mmap_attrs(), which uses
vm_insert_page(). It was implemented as
int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
struct page *page)
{
if (addr < vma->vm_start || addr >= vma->vm_end)
return -EFAULT;
if (!page_count(page))
return -EINVAL;
if (!(vma->vm_flags & VM_MIXEDMAP)) {
BUG_ON(down_read_trylock(>vm_mm->mmap_sem));
BUG_ON(vma->vm_flags & VM_PFNMAP);
vma->vm_flags |= VM_MIXEDMAP;
}
return insert_page(vma, addr, page, vma->vm_page_prot);
}
I don't recall if I just hit that BUG_ON() back then, or if I actually
understood what I was doing :-)
Today in mainline arm_iommu_mmap_attrs() uses vm_map_pages(), which
calls vm_insert_page(), and the BUG_ON() is still there.
> > > https://elixir.bootlin.com/linux/v6.1-rc6/source/mm/memory.c#L2518
> > >
> > > More importantly, it does uncondtionally set VM_PFNMAP, so clearing
> > > that does not make much sense.
> > >
> > > Patch motived by discussions around enforcing VM_PFNMAP semantics for
> > > all dma-buf users, where Thomas asked why dma helpers will work with
> > > that dma_buf_mmap() contract.
> > >
> > > References:
> > > https://lore.kernel.org/dri-devel/5c3c8d4f-2c06-9210-b00a-4d0ff6f6f...@suse.de/
> > > Cc: Laurent Pinchart
> > > Cc: Dave Airlie
> > > Cc: Maarten Lankhorst
> > > Cc: Maxime Ripard
> > > Cc: Thomas Zimmermann
> > > Cc: Sumit Semwal
> > > Cc: "Christian König"
> > > Signed-off-by: Daniel Vetter
> > > ---
> > > drivers/gpu/drm/drm_gem_dma_helper.c | 7 ++-
> > > 1 file changed, 2 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_gem_dma_helper.c
> > > b/drivers/gpu/drm/drm_gem_dma_helper.c
> > > index 1e658c448366..637a5cc62457 100644
> > > --- a/drivers/gpu/drm/drm_gem_dma_helper.c
> > > +++ b/drivers/gpu/drm/drm_gem_dma_helper.c
> > > @@ -525,13 +525,10 @@ int drm_gem_dma_mmap(struct drm_gem_dma_object
> > > *dma_obj, struct vm_area_struct *
> > > int ret;
> > > /*
> > > - * Clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
> > > - * vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
> > > - * the whole buffer.
> > > + * Set the vm_pgoff (used as a fake buffer offset by DRM) to 0 as we
> > > + * want to map the whole buffer.
> > >*/
> > > vma->vm_pgoff -= drm_vma_node_start(>vma_node);
> > > - vma->vm_flags &= ~VM_PFNMAP;
> > > - vma->vm_flags |= VM_DONTEXPAND;
> > > if (dma_obj->map_noncoherent) {
> > > vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
--
Regards,
Laurent Pinchart
Re: [PATCH] drm/dma-helpers: Don't change vma flags
On 2022-11-24 17:24, Daniel Vetter wrote:
On Thu, Nov 24, 2022 at 11:11:21AM +, Robin Murphy wrote:
On 2022-11-23 17:28, Daniel Vetter wrote:
This code was added in b65e64f7ccd4 ("drm/cma: Use
dma_mmap_writecombine() to mmap buffer"), but does not explain why
it's needed.
It should be entirely unnecessary, because remap_pfn_range(), which is
what the various dma_mmap functiosn are built on top of, does already
unconditionally adjust the vma flags:
Not all dma_mmap_*() implementations use remap_pfn_range() though. For
instance on ARM where one set of DMA ops uses vm_map_pages(), but AFAICS not
all the relevant drivers would set VM_MIXEDMAP to prevent reaching the
BUG_ON(vma->vm_flags & VM_PFNMAP) in there.
Uh a dma_mmap which does not use VM_PFNMAP has pretty good chances of
being busted, since that allows get_user_pages on these memory
allocations. And I'm really not sure that's a bright idea ...
Can you please point me at these dma-ops so that I can try and understand
what they're trying to do?
See arm_iommu_mmap_attrs(), but also one of the paths in
iommu_dma_mmap() for both arm64 and x86. These aren't using
remap_pfn_range() because they're mapping a potentially-disjoint set of
arbitrary pages, not a single physically-contiguous range. And for the
avoidance of doubt, yes, in those cases they will always be real kernel
pages. dma_mmap_attrs() can be relied upon to do the right thing for
whatever dma_alloc_attrs() did; what isn't reliable is trying to
second-guess from outside exactly what that might be.
I forgot to mention also that removing the VM_DONTEXPAND line will
seemingly just reintroduce the annoying warning spam for which we added
it in the first place (and 59f39bfa6553 does document this same reasoning).
Thanks,
Robin.
-Daniel
Robin.
https://elixir.bootlin.com/linux/v6.1-rc6/source/mm/memory.c#L2518
More importantly, it does uncondtionally set VM_PFNMAP, so clearing
that does not make much sense.
Patch motived by discussions around enforcing VM_PFNMAP semantics for
all dma-buf users, where Thomas asked why dma helpers will work with
that dma_buf_mmap() contract.
References:
https://lore.kernel.org/dri-devel/5c3c8d4f-2c06-9210-b00a-4d0ff6f6f...@suse.de/
Cc: Laurent Pinchart
Cc: Dave Airlie
Cc: Maarten Lankhorst
Cc: Maxime Ripard
Cc: Thomas Zimmermann
Cc: Sumit Semwal
Cc: "Christian K�nig"
Signed-off-by: Daniel Vetter
---
drivers/gpu/drm/drm_gem_dma_helper.c | 7 ++-
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/drm_gem_dma_helper.c
b/drivers/gpu/drm/drm_gem_dma_helper.c
index 1e658c448366..637a5cc62457 100644
--- a/drivers/gpu/drm/drm_gem_dma_helper.c
+++ b/drivers/gpu/drm/drm_gem_dma_helper.c
@@ -525,13 +525,10 @@ int drm_gem_dma_mmap(struct drm_gem_dma_object *dma_obj,
struct vm_area_struct *
int ret;
/*
-* Clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
-* vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
-* the whole buffer.
+* Set the vm_pgoff (used as a fake buffer offset by DRM) to 0 as we
+* want to map the whole buffer.
*/
vma->vm_pgoff -= drm_vma_node_start(>vma_node);
- vma->vm_flags &= ~VM_PFNMAP;
- vma->vm_flags |= VM_DONTEXPAND;
if (dma_obj->map_noncoherent) {
vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
Re: [PATCH] drm/dma-helpers: Don't change vma flags
On Thu, Nov 24, 2022 at 11:11:21AM +, Robin Murphy wrote:
> On 2022-11-23 17:28, Daniel Vetter wrote:
> > This code was added in b65e64f7ccd4 ("drm/cma: Use
> > dma_mmap_writecombine() to mmap buffer"), but does not explain why
> > it's needed.
> >
> > It should be entirely unnecessary, because remap_pfn_range(), which is
> > what the various dma_mmap functiosn are built on top of, does already
> > unconditionally adjust the vma flags:
>
> Not all dma_mmap_*() implementations use remap_pfn_range() though. For
> instance on ARM where one set of DMA ops uses vm_map_pages(), but AFAICS not
> all the relevant drivers would set VM_MIXEDMAP to prevent reaching the
> BUG_ON(vma->vm_flags & VM_PFNMAP) in there.
Uh a dma_mmap which does not use VM_PFNMAP has pretty good chances of
being busted, since that allows get_user_pages on these memory
allocations. And I'm really not sure that's a bright idea ...
Can you please point me at these dma-ops so that I can try and understand
what they're trying to do?
-Daniel
>
> Robin.
>
> > https://elixir.bootlin.com/linux/v6.1-rc6/source/mm/memory.c#L2518
> >
> > More importantly, it does uncondtionally set VM_PFNMAP, so clearing
> > that does not make much sense.
> >
> > Patch motived by discussions around enforcing VM_PFNMAP semantics for
> > all dma-buf users, where Thomas asked why dma helpers will work with
> > that dma_buf_mmap() contract.
> >
> > References:
> > https://lore.kernel.org/dri-devel/5c3c8d4f-2c06-9210-b00a-4d0ff6f6f...@suse.de/
> > Cc: Laurent Pinchart
> > Cc: Dave Airlie
> > Cc: Maarten Lankhorst
> > Cc: Maxime Ripard
> > Cc: Thomas Zimmermann
> > Cc: Sumit Semwal
> > Cc: "Christian König"
> > Signed-off-by: Daniel Vetter
> > ---
> > drivers/gpu/drm/drm_gem_dma_helper.c | 7 ++-
> > 1 file changed, 2 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_gem_dma_helper.c
> > b/drivers/gpu/drm/drm_gem_dma_helper.c
> > index 1e658c448366..637a5cc62457 100644
> > --- a/drivers/gpu/drm/drm_gem_dma_helper.c
> > +++ b/drivers/gpu/drm/drm_gem_dma_helper.c
> > @@ -525,13 +525,10 @@ int drm_gem_dma_mmap(struct drm_gem_dma_object
> > *dma_obj, struct vm_area_struct *
> > int ret;
> > /*
> > -* Clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
> > -* vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
> > -* the whole buffer.
> > +* Set the vm_pgoff (used as a fake buffer offset by DRM) to 0 as we
> > +* want to map the whole buffer.
> > */
> > vma->vm_pgoff -= drm_vma_node_start(>vma_node);
> > - vma->vm_flags &= ~VM_PFNMAP;
> > - vma->vm_flags |= VM_DONTEXPAND;
> > if (dma_obj->map_noncoherent) {
> > vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
Re: [PATCH] drm/dma-helpers: Don't change vma flags
On 2022-11-23 17:28, Daniel Vetter wrote:
This code was added in b65e64f7ccd4 ("drm/cma: Use
dma_mmap_writecombine() to mmap buffer"), but does not explain why
it's needed.
It should be entirely unnecessary, because remap_pfn_range(), which is
what the various dma_mmap functiosn are built on top of, does already
unconditionally adjust the vma flags:
Not all dma_mmap_*() implementations use remap_pfn_range() though. For
instance on ARM where one set of DMA ops uses vm_map_pages(), but AFAICS
not all the relevant drivers would set VM_MIXEDMAP to prevent reaching
the BUG_ON(vma->vm_flags & VM_PFNMAP) in there.
Robin.
https://elixir.bootlin.com/linux/v6.1-rc6/source/mm/memory.c#L2518
More importantly, it does uncondtionally set VM_PFNMAP, so clearing
that does not make much sense.
Patch motived by discussions around enforcing VM_PFNMAP semantics for
all dma-buf users, where Thomas asked why dma helpers will work with
that dma_buf_mmap() contract.
References:
https://lore.kernel.org/dri-devel/5c3c8d4f-2c06-9210-b00a-4d0ff6f6f...@suse.de/
Cc: Laurent Pinchart
Cc: Dave Airlie
Cc: Maarten Lankhorst
Cc: Maxime Ripard
Cc: Thomas Zimmermann
Cc: Sumit Semwal
Cc: "Christian König"
Signed-off-by: Daniel Vetter
---
drivers/gpu/drm/drm_gem_dma_helper.c | 7 ++-
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/drm_gem_dma_helper.c
b/drivers/gpu/drm/drm_gem_dma_helper.c
index 1e658c448366..637a5cc62457 100644
--- a/drivers/gpu/drm/drm_gem_dma_helper.c
+++ b/drivers/gpu/drm/drm_gem_dma_helper.c
@@ -525,13 +525,10 @@ int drm_gem_dma_mmap(struct drm_gem_dma_object *dma_obj,
struct vm_area_struct *
int ret;
/*
-* Clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
-* vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
-* the whole buffer.
+* Set the vm_pgoff (used as a fake buffer offset by DRM) to 0 as we
+* want to map the whole buffer.
*/
vma->vm_pgoff -= drm_vma_node_start(>vma_node);
- vma->vm_flags &= ~VM_PFNMAP;
- vma->vm_flags |= VM_DONTEXPAND;
if (dma_obj->map_noncoherent) {
vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
[PATCH] drm/dma-helpers: Don't change vma flags
This code was added in b65e64f7ccd4 ("drm/cma: Use
dma_mmap_writecombine() to mmap buffer"), but does not explain why
it's needed.
It should be entirely unnecessary, because remap_pfn_range(), which is
what the various dma_mmap functiosn are built on top of, does already
unconditionally adjust the vma flags:
https://elixir.bootlin.com/linux/v6.1-rc6/source/mm/memory.c#L2518
More importantly, it does uncondtionally set VM_PFNMAP, so clearing
that does not make much sense.
Patch motived by discussions around enforcing VM_PFNMAP semantics for
all dma-buf users, where Thomas asked why dma helpers will work with
that dma_buf_mmap() contract.
References:
https://lore.kernel.org/dri-devel/5c3c8d4f-2c06-9210-b00a-4d0ff6f6f...@suse.de/
Cc: Laurent Pinchart
Cc: Dave Airlie
Cc: Maarten Lankhorst
Cc: Maxime Ripard
Cc: Thomas Zimmermann
Cc: Sumit Semwal
Cc: "Christian König"
Signed-off-by: Daniel Vetter
---
drivers/gpu/drm/drm_gem_dma_helper.c | 7 ++-
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/drm_gem_dma_helper.c
b/drivers/gpu/drm/drm_gem_dma_helper.c
index 1e658c448366..637a5cc62457 100644
--- a/drivers/gpu/drm/drm_gem_dma_helper.c
+++ b/drivers/gpu/drm/drm_gem_dma_helper.c
@@ -525,13 +525,10 @@ int drm_gem_dma_mmap(struct drm_gem_dma_object *dma_obj,
struct vm_area_struct *
int ret;
/*
-* Clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
-* vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
-* the whole buffer.
+* Set the vm_pgoff (used as a fake buffer offset by DRM) to 0 as we
+* want to map the whole buffer.
*/
vma->vm_pgoff -= drm_vma_node_start(>vma_node);
- vma->vm_flags &= ~VM_PFNMAP;
- vma->vm_flags |= VM_DONTEXPAND;
if (dma_obj->map_noncoherent) {
vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
--
2.37.2
