* [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region
@ 2020-05-21 1:35 Dan Williams
2020-05-21 2:26 ` Matthew Wilcox
0 siblings, 1 reply; 6+ messages in thread
From: Dan Williams @ 2020-05-21 1:35 UTC (permalink / raw)
To: gregkh
Cc: Arnd Bergmann, Ingo Molnar, Kees Cook, Matthew Wilcox,
Russell King, Andrew Morton, akpm, linux-kernel, linux-mm
Close the hole of holding a mapping over kernel driver takeover event of
a given address range.
Commit 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
introduced CONFIG_IO_STRICT_DEVMEM with the goal of protecting the
kernel against scenarios where a /dev/mem user tramples memory that a
kernel driver owns. However, this protection only prevents *new* read(),
write() and mmap() requests. Established mappings prior to the driver
calling request_mem_region() are left alone.
Especially with persistent memory, and the core kernel metadata that is
stored there, there are plentiful scenarios for a /dev/mem user to
violate the expectations of the driver and cause amplified damage.
Teach request_mem_region() to find and shoot down active /dev/mem
mappings that it believes it has successfully claimed for the exclusive
use of the driver. Effectively a driver call to request_mem_region()
becomes a hole-punch on the /dev/mem device.
The typical usage of unmap_mapping_range() is part of
truncate_pagecache() to punch a hole in a file, but in this case the
implementation is only doing the "first half" of a hole punch. Namely it
is just evacuating current established mappings of the "hole", and it
relies on the fact that /dev/mem establishes mappings in terms of
absolute physical address offsets. Once existing mmap users are
invalidated they can attempt to re-establish the mapping, or attempt to
continue issuing read(2) / write(2) to the invalidated extent, but they
will then be subject to the CONFIG_IO_STRICT_DEVMEM checking that can
block those subsequent accesses.
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
Changes since v2 [1]:
- Fix smp_wmb() placement relative to publishing write (Matthew)
[1]: http://lore.kernel.org/r/158987153989.4000084.17143582803685077783.stgit@dwillia2-desk3.amr.corp.intel.com
drivers/char/mem.c | 104 +++++++++++++++++++++++++++++++++++++++++++-
include/linux/ioport.h | 6 +++
include/uapi/linux/magic.h | 1
kernel/resource.c | 5 ++
4 files changed, 114 insertions(+), 2 deletions(-)
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 43dd0891ca1e..46bea7a25983 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -31,11 +31,15 @@
#include <linux/uio.h>
#include <linux/uaccess.h>
#include <linux/security.h>
+#include <linux/pseudo_fs.h>
+#include <uapi/linux/magic.h>
+#include <linux/mount.h>
#ifdef CONFIG_IA64
# include <linux/efi.h>
#endif
+#define DEVMEM_MINOR 1
#define DEVPORT_MINOR 4
static inline unsigned long size_inside_page(unsigned long start,
@@ -805,12 +809,66 @@ static loff_t memory_lseek(struct file *file, loff_t offset, int orig)
return ret;
}
+static struct inode *devmem_inode;
+
+#ifdef CONFIG_IO_STRICT_DEVMEM
+void revoke_devmem(struct resource *res)
+{
+ struct inode *inode = READ_ONCE(devmem_inode);
+
+ /*
+ * Check that the initialization has completed. Losing the race
+ * is ok because it means drivers are claiming resources before
+ * the fs_initcall level of init and prevent /dev/mem from
+ * establishing mappings.
+ */
+ smp_rmb();
+ if (!inode)
+ return;
+
+ /*
+ * The expectation is that the driver has successfully marked
+ * the resource busy by this point, so devmem_is_allowed()
+ * should start returning false, however for performance this
+ * does not iterate the entire resource range.
+ */
+ if (devmem_is_allowed(PHYS_PFN(res->start)) &&
+ devmem_is_allowed(PHYS_PFN(res->end))) {
+ /*
+ * *cringe* iomem=relaxed says "go ahead, what's the
+ * worst that can happen?"
+ */
+ return;
+ }
+
+ unmap_mapping_range(inode->i_mapping, res->start, resource_size(res), 1);
+}
+#endif
+
static int open_port(struct inode *inode, struct file *filp)
{
+ int rc;
+
if (!capable(CAP_SYS_RAWIO))
return -EPERM;
- return security_locked_down(LOCKDOWN_DEV_MEM);
+ rc = security_locked_down(LOCKDOWN_DEV_MEM);
+ if (rc)
+ return rc;
+
+ if (iminor(inode) != DEVMEM_MINOR)
+ return 0;
+
+ /*
+ * Use a unified address space to have a single point to manage
+ * revocations when drivers want to take over a /dev/mem mapped
+ * range.
+ */
+ inode->i_mapping = devmem_inode->i_mapping;
+ inode->i_mapping->host = devmem_inode;
+ filp->f_mapping = inode->i_mapping;
+
+ return 0;
}
#define zero_lseek null_lseek
@@ -885,7 +943,7 @@ static const struct memdev {
fmode_t fmode;
} devlist[] = {
#ifdef CONFIG_DEVMEM
- [1] = { "mem", 0, &mem_fops, FMODE_UNSIGNED_OFFSET },
+ [DEVMEM_MINOR] = { "mem", 0, &mem_fops, FMODE_UNSIGNED_OFFSET },
#endif
#ifdef CONFIG_DEVKMEM
[2] = { "kmem", 0, &kmem_fops, FMODE_UNSIGNED_OFFSET },
@@ -939,6 +997,46 @@ static char *mem_devnode(struct device *dev, umode_t *mode)
static struct class *mem_class;
+static int devmem_fs_init_fs_context(struct fs_context *fc)
+{
+ return init_pseudo(fc, DEVMEM_MAGIC) ? 0 : -ENOMEM;
+}
+
+static struct file_system_type devmem_fs_type = {
+ .name = "devmem",
+ .owner = THIS_MODULE,
+ .init_fs_context = devmem_fs_init_fs_context,
+ .kill_sb = kill_anon_super,
+};
+
+static int devmem_init_inode(void)
+{
+ static struct vfsmount *devmem_vfs_mount;
+ static int devmem_fs_cnt;
+ struct inode *inode;
+ int rc;
+
+ rc = simple_pin_fs(&devmem_fs_type, &devmem_vfs_mount, &devmem_fs_cnt);
+ if (rc < 0) {
+ pr_err("Cannot mount /dev/mem pseudo filesystem: %d\n", rc);
+ return rc;
+ }
+
+ inode = alloc_anon_inode(devmem_vfs_mount->mnt_sb);
+ if (IS_ERR(inode)) {
+ rc = PTR_ERR(inode);
+ pr_err("Cannot allocate inode for /dev/mem: %d\n", rc);
+ simple_release_fs(&devmem_vfs_mount, &devmem_fs_cnt);
+ return rc;
+ }
+
+ /* publish /dev/mem initialized */
+ smp_wmb();
+ WRITE_ONCE(devmem_inode, inode);
+
+ return 0;
+}
+
static int __init chr_dev_init(void)
{
int minor;
@@ -960,6 +1058,8 @@ static int __init chr_dev_init(void)
*/
if ((minor == DEVPORT_MINOR) && !arch_has_dev_port())
continue;
+ if ((minor == DEVMEM_MINOR) && devmem_init_inode() != 0)
+ continue;
device_create(mem_class, NULL, MKDEV(MEM_MAJOR, minor),
NULL, devlist[minor].name);
diff --git a/include/linux/ioport.h b/include/linux/ioport.h
index a9b9170b5dd2..6c3eca90cbc4 100644
--- a/include/linux/ioport.h
+++ b/include/linux/ioport.h
@@ -301,5 +301,11 @@ struct resource *devm_request_free_mem_region(struct device *dev,
struct resource *request_free_mem_region(struct resource *base,
unsigned long size, const char *name);
+#ifdef CONFIG_IO_STRICT_DEVMEM
+void revoke_devmem(struct resource *res);
+#else
+static inline void revoke_devmem(struct resource *res) { };
+#endif
+
#endif /* __ASSEMBLY__ */
#endif /* _LINUX_IOPORT_H */
diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index d78064007b17..f3956fc11de6 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -94,6 +94,7 @@
#define BALLOON_KVM_MAGIC 0x13661366
#define ZSMALLOC_MAGIC 0x58295829
#define DMA_BUF_MAGIC 0x444d4142 /* "DMAB" */
+#define DEVMEM_MAGIC 0x454d444d /* "DMEM" */
#define Z3FOLD_MAGIC 0x33
#define PPC_CMM_MAGIC 0xc7571590
diff --git a/kernel/resource.c b/kernel/resource.c
index 76036a41143b..841737bbda9e 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -1126,6 +1126,7 @@ struct resource * __request_region(struct resource *parent,
{
DECLARE_WAITQUEUE(wait, current);
struct resource *res = alloc_resource(GFP_KERNEL);
+ struct resource *orig_parent = parent;
if (!res)
return NULL;
@@ -1176,6 +1177,10 @@ struct resource * __request_region(struct resource *parent,
break;
}
write_unlock(&resource_lock);
+
+ if (res && orig_parent == &iomem_resource)
+ revoke_devmem(res);
+
return res;
}
EXPORT_SYMBOL(__request_region);
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region
2020-05-21 1:35 [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region Dan Williams
@ 2020-05-21 2:26 ` Matthew Wilcox
2020-05-21 4:37 ` Dan Williams
0 siblings, 1 reply; 6+ messages in thread
From: Matthew Wilcox @ 2020-05-21 2:26 UTC (permalink / raw)
To: Dan Williams
Cc: gregkh, Arnd Bergmann, Ingo Molnar, Kees Cook, Russell King,
Andrew Morton, linux-kernel, linux-mm
On Wed, May 20, 2020 at 06:35:25PM -0700, Dan Williams wrote:
> +static struct inode *devmem_inode;
> +
> +#ifdef CONFIG_IO_STRICT_DEVMEM
> +void revoke_devmem(struct resource *res)
> +{
> + struct inode *inode = READ_ONCE(devmem_inode);
> +
> + /*
> + * Check that the initialization has completed. Losing the race
> + * is ok because it means drivers are claiming resources before
> + * the fs_initcall level of init and prevent /dev/mem from
> + * establishing mappings.
> + */
> + smp_rmb();
> + if (!inode)
> + return;
But we don't need the smp_rmb() here, right? READ_ONCE and WRITE_ONCE
are a DATA DEPENDENCY barrier (in Documentation/memory-barriers.txt parlance)
so the smp_rmb() is superfluous ...
> + /*
> + * Use a unified address space to have a single point to manage
> + * revocations when drivers want to take over a /dev/mem mapped
> + * range.
> + */
> + inode->i_mapping = devmem_inode->i_mapping;
> + inode->i_mapping->host = devmem_inode;
umm ... devmem_inode->i_mapping->host doesn't already point to devmem_inode?
> +
> + /* publish /dev/mem initialized */
> + smp_wmb();
> + WRITE_ONCE(devmem_inode, inode);
As above, unnecessary barrier, I think.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region
2020-05-21 2:26 ` Matthew Wilcox
@ 2020-05-21 4:37 ` Dan Williams
2020-05-21 4:39 ` Dan Williams
0 siblings, 1 reply; 6+ messages in thread
From: Dan Williams @ 2020-05-21 4:37 UTC (permalink / raw)
To: Matthew Wilcox
Cc: Greg KH, Arnd Bergmann, Ingo Molnar, Kees Cook, Russell King,
Andrew Morton, Linux Kernel Mailing List, Linux MM
On Wed, May 20, 2020 at 7:26 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, May 20, 2020 at 06:35:25PM -0700, Dan Williams wrote:
> > +static struct inode *devmem_inode;
> > +
> > +#ifdef CONFIG_IO_STRICT_DEVMEM
> > +void revoke_devmem(struct resource *res)
> > +{
> > + struct inode *inode = READ_ONCE(devmem_inode);
> > +
> > + /*
> > + * Check that the initialization has completed. Losing the race
> > + * is ok because it means drivers are claiming resources before
> > + * the fs_initcall level of init and prevent /dev/mem from
> > + * establishing mappings.
> > + */
> > + smp_rmb();
> > + if (!inode)
> > + return;
>
> But we don't need the smp_rmb() here, right? READ_ONCE and WRITE_ONCE
> are a DATA DEPENDENCY barrier (in Documentation/memory-barriers.txt parlance)
> so the smp_rmb() is superfluous ...
Is it? I did not grok that from Documentation/memory-barriers.txt.
READ_ONCE and WRITE_ONCE are certainly ordered with respect to each
other in the same function, but I thought they still depend on
barriers for smp ordering?
>
> > + /*
> > + * Use a unified address space to have a single point to manage
> > + * revocations when drivers want to take over a /dev/mem mapped
> > + * range.
> > + */
> > + inode->i_mapping = devmem_inode->i_mapping;
> > + inode->i_mapping->host = devmem_inode;
>
> umm ... devmem_inode->i_mapping->host doesn't already point to devmem_inode?
Not if inode is coming from:
mknod ./newmem c 1 1
...that's the problem that a unified inode solves. You can mknod all
you want, but mapping and mapping->host will point to a common
instance.
>
> > +
> > + /* publish /dev/mem initialized */
> > + smp_wmb();
> > + WRITE_ONCE(devmem_inode, inode);
>
> As above, unnecessary barrier, I think.
Well, if you're not sure, how sure should I be?
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region
2020-05-21 4:37 ` Dan Williams
@ 2020-05-21 4:39 ` Dan Williams
2020-05-21 11:41 ` Matthew Wilcox
0 siblings, 1 reply; 6+ messages in thread
From: Dan Williams @ 2020-05-21 4:39 UTC (permalink / raw)
To: Matthew Wilcox
Cc: Greg KH, Arnd Bergmann, Ingo Molnar, Kees Cook, Russell King,
Andrew Morton, Linux Kernel Mailing List, Linux MM
On Wed, May 20, 2020 at 9:37 PM Dan Williams <dan.j.williams@intel.com> wrote:
>
> On Wed, May 20, 2020 at 7:26 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Wed, May 20, 2020 at 06:35:25PM -0700, Dan Williams wrote:
> > > +static struct inode *devmem_inode;
> > > +
> > > +#ifdef CONFIG_IO_STRICT_DEVMEM
> > > +void revoke_devmem(struct resource *res)
> > > +{
> > > + struct inode *inode = READ_ONCE(devmem_inode);
> > > +
> > > + /*
> > > + * Check that the initialization has completed. Losing the race
> > > + * is ok because it means drivers are claiming resources before
> > > + * the fs_initcall level of init and prevent /dev/mem from
> > > + * establishing mappings.
> > > + */
> > > + smp_rmb();
> > > + if (!inode)
> > > + return;
> >
> > But we don't need the smp_rmb() here, right? READ_ONCE and WRITE_ONCE
> > are a DATA DEPENDENCY barrier (in Documentation/memory-barriers.txt parlance)
> > so the smp_rmb() is superfluous ...
>
> Is it? I did not grok that from Documentation/memory-barriers.txt.
> READ_ONCE and WRITE_ONCE are certainly ordered with respect to each
> other in the same function, but I thought they still depend on
> barriers for smp ordering?
>
> >
> > > + /*
> > > + * Use a unified address space to have a single point to manage
> > > + * revocations when drivers want to take over a /dev/mem mapped
> > > + * range.
> > > + */
> > > + inode->i_mapping = devmem_inode->i_mapping;
> > > + inode->i_mapping->host = devmem_inode;
> >
> > umm ... devmem_inode->i_mapping->host doesn't already point to devmem_inode?
>
> Not if inode is coming from:
>
> mknod ./newmem c 1 1
>
> ...that's the problem that a unified inode solves. You can mknod all
> you want, but mapping and mapping->host will point to a common
> instance.
>
> >
> > > +
> > > + /* publish /dev/mem initialized */
> > > + smp_wmb();
> > > + WRITE_ONCE(devmem_inode, inode);
> >
> > As above, unnecessary barrier, I think.
>
> Well, if you're not sure, how sure should I be?
I'm pretty sure they are needed, because I need the prior writes to
initialize the inode to be fenced before the final write to publish
the inode. I don't think WRITE_ONCE() enforces that prior writes have
completed.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region
2020-05-21 4:39 ` Dan Williams
@ 2020-05-21 11:41 ` Matthew Wilcox
2020-05-21 18:21 ` Dan Williams
0 siblings, 1 reply; 6+ messages in thread
From: Matthew Wilcox @ 2020-05-21 11:41 UTC (permalink / raw)
To: Dan Williams
Cc: Greg KH, Arnd Bergmann, Ingo Molnar, Kees Cook, Russell King,
Andrew Morton, Linux Kernel Mailing List, Linux MM
On Wed, May 20, 2020 at 09:39:49PM -0700, Dan Williams wrote:
> On Wed, May 20, 2020 at 9:37 PM Dan Williams <dan.j.williams@intel.com> wrote:
> > On Wed, May 20, 2020 at 7:26 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > On Wed, May 20, 2020 at 06:35:25PM -0700, Dan Williams wrote:
> > > > +static struct inode *devmem_inode;
> > > > +
> > > > +#ifdef CONFIG_IO_STRICT_DEVMEM
> > > > +void revoke_devmem(struct resource *res)
> > > > +{
> > > > + struct inode *inode = READ_ONCE(devmem_inode);
> > > > +
> > > > + /*
> > > > + * Check that the initialization has completed. Losing the race
> > > > + * is ok because it means drivers are claiming resources before
> > > > + * the fs_initcall level of init and prevent /dev/mem from
> > > > + * establishing mappings.
> > > > + */
> > > > + smp_rmb();
> > > > + if (!inode)
> > > > + return;
> > >
> > > But we don't need the smp_rmb() here, right? READ_ONCE and WRITE_ONCE
> > > are a DATA DEPENDENCY barrier (in Documentation/memory-barriers.txt parlance)
> > > so the smp_rmb() is superfluous ...
> >
> > Is it? I did not grok that from Documentation/memory-barriers.txt.
> > READ_ONCE and WRITE_ONCE are certainly ordered with respect to each
> > other in the same function, but I thought they still depend on
> > barriers for smp ordering?
> >
> > > > +
> > > > + /* publish /dev/mem initialized */
> > > > + smp_wmb();
> > > > + WRITE_ONCE(devmem_inode, inode);
> > >
> > > As above, unnecessary barrier, I think.
> >
> > Well, if you're not sure, how sure should I be?
>
> I'm pretty sure they are needed, because I need the prior writes to
> initialize the inode to be fenced before the final write to publish
> the inode. I don't think WRITE_ONCE() enforces that prior writes have
> completed.
Completed, no, but I think it does enforce that they're visible to other
CPUs before this write is visible to other CPUs.
I'll quote relevant bits from the document ...
(2) Data dependency barriers.
A data dependency barrier is a weaker form of read barrier. In the case
where two loads are performed such that the second depends on the result
of the first (eg: the first load retrieves the address to which the second
load will be directed), a data dependency barrier would be required to
make sure that the target of the second load is updated after the address
obtained by the first load is accessed.
[...]
SMP BARRIER PAIRING
-------------------
[...]
CPU 1 CPU 2
=============== ===============================
a = 1;
<write barrier>
WRITE_ONCE(b, &a); x = READ_ONCE(b);
<data dependency barrier>
y = *x;
> > >
> > > > + /*
> > > > + * Use a unified address space to have a single point to manage
> > > > + * revocations when drivers want to take over a /dev/mem mapped
> > > > + * range.
> > > > + */
> > > > + inode->i_mapping = devmem_inode->i_mapping;
> > > > + inode->i_mapping->host = devmem_inode;
> > >
> > > umm ... devmem_inode->i_mapping->host doesn't already point to devmem_inode?
> >
> > Not if inode is coming from:
> >
> > mknod ./newmem c 1 1
> >
> > ...that's the problem that a unified inode solves. You can mknod all
> > you want, but mapping and mapping->host will point to a common
> > instance.
I don't think I explained myself well enough.
When we initialise devmem_inode, does devmem_inode->i_mapping->host point
to somewhere other than devmem_inode?
I appreciate in this function, inode->i_mapping->host will point to inode.
But we're now changing i_mapping to be devmem_inode's i_mapping. Why
do we need to change devmem_inode's i_mapping->host pointer?
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region
2020-05-21 11:41 ` Matthew Wilcox
@ 2020-05-21 18:21 ` Dan Williams
0 siblings, 0 replies; 6+ messages in thread
From: Dan Williams @ 2020-05-21 18:21 UTC (permalink / raw)
To: Matthew Wilcox
Cc: Greg KH, Arnd Bergmann, Ingo Molnar, Kees Cook, Russell King,
Andrew Morton, Linux Kernel Mailing List, Linux MM
On Thu, May 21, 2020 at 4:41 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, May 20, 2020 at 09:39:49PM -0700, Dan Williams wrote:
> > On Wed, May 20, 2020 at 9:37 PM Dan Williams <dan.j.williams@intel.com> wrote:
> > > On Wed, May 20, 2020 at 7:26 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > > On Wed, May 20, 2020 at 06:35:25PM -0700, Dan Williams wrote:
> > > > > +static struct inode *devmem_inode;
> > > > > +
> > > > > +#ifdef CONFIG_IO_STRICT_DEVMEM
> > > > > +void revoke_devmem(struct resource *res)
> > > > > +{
> > > > > + struct inode *inode = READ_ONCE(devmem_inode);
> > > > > +
> > > > > + /*
> > > > > + * Check that the initialization has completed. Losing the race
> > > > > + * is ok because it means drivers are claiming resources before
> > > > > + * the fs_initcall level of init and prevent /dev/mem from
> > > > > + * establishing mappings.
> > > > > + */
> > > > > + smp_rmb();
> > > > > + if (!inode)
> > > > > + return;
> > > >
> > > > But we don't need the smp_rmb() here, right? READ_ONCE and WRITE_ONCE
> > > > are a DATA DEPENDENCY barrier (in Documentation/memory-barriers.txt parlance)
> > > > so the smp_rmb() is superfluous ...
> > >
> > > Is it? I did not grok that from Documentation/memory-barriers.txt.
> > > READ_ONCE and WRITE_ONCE are certainly ordered with respect to each
> > > other in the same function, but I thought they still depend on
> > > barriers for smp ordering?
> > >
> > > > > +
> > > > > + /* publish /dev/mem initialized */
> > > > > + smp_wmb();
> > > > > + WRITE_ONCE(devmem_inode, inode);
> > > >
> > > > As above, unnecessary barrier, I think.
> > >
> > > Well, if you're not sure, how sure should I be?
> >
> > I'm pretty sure they are needed, because I need the prior writes to
> > initialize the inode to be fenced before the final write to publish
> > the inode. I don't think WRITE_ONCE() enforces that prior writes have
> > completed.
>
> Completed, no, but I think it does enforce that they're visible to other
> CPUs before this write is visible to other CPUs.
>
> I'll quote relevant bits from the document ...
>
> (2) Data dependency barriers.
>
> A data dependency barrier is a weaker form of read barrier. In the case
> where two loads are performed such that the second depends on the result
> of the first (eg: the first load retrieves the address to which the second
> load will be directed), a data dependency barrier would be required to
> make sure that the target of the second load is updated after the address
> obtained by the first load is accessed.
>
> [...]
> SMP BARRIER PAIRING
> -------------------
> [...]
> CPU 1 CPU 2
> =============== ===============================
> a = 1;
> <write barrier>
> WRITE_ONCE(b, &a); x = READ_ONCE(b);
> <data dependency barrier>
> y = *x;
>
Oh, I read those <* barrier> lines as a requirement not an implied
side effect of READ/WRITE_ONCE(). I can see that WRITE_ONCE() is
effectively a barrier() and READ_ONCE() includes
smp_read_barrier_depends(). I'll drop.
>
> > > >
> > > > > + /*
> > > > > + * Use a unified address space to have a single point to manage
> > > > > + * revocations when drivers want to take over a /dev/mem mapped
> > > > > + * range.
> > > > > + */
> > > > > + inode->i_mapping = devmem_inode->i_mapping;
> > > > > + inode->i_mapping->host = devmem_inode;
> > > >
> > > > umm ... devmem_inode->i_mapping->host doesn't already point to devmem_inode?
> > >
> > > Not if inode is coming from:
> > >
> > > mknod ./newmem c 1 1
> > >
> > > ...that's the problem that a unified inode solves. You can mknod all
> > > you want, but mapping and mapping->host will point to a common
> > > instance.
>
> I don't think I explained myself well enough.
>
> When we initialise devmem_inode, does devmem_inode->i_mapping->host point
> to somewhere other than devmem_inode?
>
> I appreciate in this function, inode->i_mapping->host will point to inode.
> But we're now changing i_mapping to be devmem_inode's i_mapping. Why
> do we need to change devmem_inode's i_mapping->host pointer?
>
Yeah, mistook your comment. The setting of ->host is indeed redundant.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2020-05-21 18:21 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-05-21 1:35 [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region Dan Williams
2020-05-21 2:26 ` Matthew Wilcox
2020-05-21 4:37 ` Dan Williams
2020-05-21 4:39 ` Dan Williams
2020-05-21 11:41 ` Matthew Wilcox
2020-05-21 18:21 ` Dan Williams
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).