Re: [PATCH 1/2] i915: convert to new mount API

[Chris Wilson]

Quoting Sergey Senozhatsky (2019-08-02 13:39:55)
> tmpfs does not set ->remount_fs() anymore and its users need
> to be converted to new mount API.
> 
>  BUG: kernel NULL pointer dereference, address: 0000000000000000
>  PF: supervisor instruction fetch in kernel mode
>  PF: error_code(0x0010) - not-present page
>  RIP: 0010:0x0
>  Code: Bad RIP value.
>  Call Trace:
>   i915_gemfs_init+0x6e/0xa0 [i915]
>   i915_gem_init_early+0x76/0x90 [i915]
>   i915_driver_probe+0x30a/0x1640 [i915]
>   ? kernfs_activate+0x5a/0x80
>   ? kernfs_add_one+0xdd/0x130
>   pci_device_probe+0x9e/0x110
>   really_probe+0xce/0x230
>   driver_probe_device+0x4b/0xc0
>   device_driver_attach+0x4e/0x60
>   __driver_attach+0x47/0xb0
>   ? device_driver_attach+0x60/0x60
>   bus_for_each_dev+0x61/0x90
>   bus_add_driver+0x167/0x1b0
>   driver_register+0x67/0xaa
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> ---
>  drivers/gpu/drm/i915/gem/i915_gemfs.c | 28 ++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gemfs.c b/drivers/gpu/drm/i915/gem/i915_gemfs.c
> index 099f3397aada..cf05ba72df9d 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gemfs.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gemfs.c
> @@ -7,14 +7,17 @@
>  #include <linux/fs.h>
>  #include <linux/mount.h>
>  #include <linux/pagemap.h>
> +#include <linux/fs_context.h>
>  
>  #include "i915_drv.h"
>  #include "i915_gemfs.h"
>  
>  int i915_gemfs_init(struct drm_i915_private *i915)
>  {
> +       struct fs_context *fc = NULL;
>         struct file_system_type *type;
>         struct vfsmount *gemfs;
> +       bool ok = true;

Start with ok = false, we only need to set to true if we succeed in
reconfiguring.

>         type = get_fs_type("tmpfs");
>         if (!type)
> @@ -36,18 +39,29 @@ int i915_gemfs_init(struct drm_i915_private *i915)
>                 struct super_block *sb = gemfs->mnt_sb;
>                 /* FIXME: Disabled until we get W/A for read BW issue. */
>                 char options[] = "huge=never";
> -               int flags = 0;
> -               int err;

Hmm, we could avoid this if we used vfs_kernel_mount() directly rather
than the kern_mount wrapper, as then we pass options through to
parse_monotithic_mount_data(). Or am I barking up the wrong tree?

>  
> -               err = sb->s_op->remount_fs(sb, &flags, options);
> -               if (err) {
> -                       kern_unmount(gemfs);
> -                       return err;
> +               fc = fs_context_for_reconfigure(sb->s_root, 0, 0);
> +               if (IS_ERR(fc)) {
> +                       ok = false;
> +                       goto out;
>                 }
> +
> +               if (!fc->ops->parse_monolithic ||
> +                               fc->ops->parse_monolithic(fc, options)) {

checkpatch.pl will complain that this should line up with the '('

> +                       ok = false;
> +                       goto out;
> +               }
> +
> +               if (!fc->ops->reconfigure || fc->ops->reconfigure(fc))
> +                       ok = false;
>         }
>  
> +out:
> +       if (!ok)
> +               pr_err("i915 gemfs reconfiguration failed\n");

Let's make it a bit more user friendly,

dev_err(i915->drm.dev,
	"Unable to reconfigure internal shmemfs for preferred"
	" allocation strategy; continuing, but performance may suffer.\n");

Assuming that we can't just use vfs_kern_mount() instead, with the nits
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
-Chris