On Thu, 28 Apr 2016, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> copy_params uses GFP_NOIO for explicit allocation requests because this
> might be called from the suspend path. To quote Mikulas:
> : The LVM tool calls suspend and resume ioctls on device mapper block
> : devices.
> :
> : When a device is suspended, any bio sent to the device is held. If the
> : resume ioctl did GFP_KERNEL allocation, the allocation could get stuck
> : trying to write some dirty cached pages to the suspended device.
> :
> : The LVM tool and the dmeventd daemon use mlock to lock its address space,
> : so the copy_from_user/copy_to_user call cannot trigger a page fault.
>
> Relying on the mlock is quite fragile and we have a better way in kernel
> to enfore NOIO which is already used for the vmalloc fallback. Just use
> memalloc_noio_{save,restore} around the whole copy_params function which
> will force the same also to the page fult paths via copy_{from,to}_user.
The userspace memory is locked, so we don't need to use memalloc_noio_save
around copy_from_user. If the memory weren't locked, memalloc_noio_save
wouldn't help us to prevent the IO.
We don't need this change (unless you show that it fixes real bug).
Mikulas
> While we are there we can also remove __GFP_NOMEMALLOC because copy_params
> is never called from MEMALLOC context (e.g. during the reclaim).
>
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Cc: dm-devel@xxxxxxxxxx
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> drivers/md/dm-ioctl.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> index 2c7ca258c4e4..fe0b57d7573c 100644
> --- a/drivers/md/dm-ioctl.c
> +++ b/drivers/md/dm-ioctl.c
> @@ -1715,16 +1715,13 @@ static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl *param_kern
> */
> dmi = NULL;
> if (param_kernel->data_size <= KMALLOC_MAX_SIZE) {
> - dmi = kmalloc(param_kernel->data_size, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
> + dmi = kmalloc(param_kernel->data_size, GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN);
> if (dmi)
> *param_flags |= DM_PARAMS_KMALLOC;
> }
>
> if (!dmi) {
> - unsigned noio_flag;
> - noio_flag = memalloc_noio_save();
> - dmi = __vmalloc(param_kernel->data_size, GFP_NOIO | __GFP_HIGH | __GFP_HIGHMEM, PAGE_KERNEL);
> - memalloc_noio_restore(noio_flag);
> + dmi = __vmalloc(param_kernel->data_size, GFP_KERNEL | __GFP_HIGH | __GFP_HIGHMEM, PAGE_KERNEL);
> if (dmi)
> *param_flags |= DM_PARAMS_VMALLOC;
> }
> @@ -1801,6 +1798,7 @@ static int ctl_ioctl(uint command, struct dm_ioctl __user *user)
> ioctl_fn fn = NULL;
> size_t input_param_size;
> struct dm_ioctl param_kernel;
> + unsigned noio_flag;
>
> /* only root can play with this */
> if (!capable(CAP_SYS_ADMIN))
> @@ -1832,9 +1830,12 @@ static int ctl_ioctl(uint command, struct dm_ioctl __user *user)
> }
>
> /*
> - * Copy the parameters into kernel space.
> + * Copy the parameters into kernel space. Make sure that no IO is triggered
> + * from the allocation paths because this might be called during the suspend.
> */
> + noio_flag = memalloc_noio_save();
> r = copy_params(user, ¶m_kernel, ioctl_flags, ¶m, ¶m_flags);
> + memalloc_noio_restore(noio_flag);
>
> if (r)
> return r;
> --
> 2.8.0.rc3
>
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