Re: [PATCH] ipc/shm.c add ->pagesize function to shm_vm_ops

[Michal Hocko]

On Fri 27-07-18 15:17:27, Jane Chu wrote:
> Commit 05ea88608d4e13 (mm, hugetlbfs: introduce ->pagesize() to
> vm_operations_struct) adds a new ->pagesize() function to
> hugetlb_vm_ops, intended to cover all hugetlbfs backed files.
> 
> With System V shared memory model, if "huge page" is specified,
> the "shared memory" is backed by hugetlbfs files, but the mappings
> initiated via shmget/shmat have their original vm_ops overwritten
> with shm_vm_ops, so we need to add a ->pagesize function to shm_vm_ops.
> Otherwise, vma_kernel_pagesize() returns PAGE_SIZE given a hugetlbfs
> backed vma, result in below BUG:
> 
> fs/hugetlbfs/inode.c
>         443             if (unlikely(page_mapped(page))) {
>         444                     BUG_ON(truncate_op);
> 
> [  242.268342] hugetlbfs: oracle (4592): Using mlock ulimits for SHM_HUGETLB is deprecated
> [  282.653208] ------------[ cut here ]------------
> [  282.708447] kernel BUG at fs/hugetlbfs/inode.c:444!
> [  282.818957] Modules linked in: nfsv3 rpcsec_gss_krb5 nfsv4 ...
> [  284.025873] CPU: 35 PID: 5583 Comm: oracle_5583_sbt Not tainted 4.14.35-1829.el7uek.x86_64 #2
> [  284.246609] task: ffff9bf0507aaf80 task.stack: ffffa9e625628000
> [  284.317455] RIP: 0010:remove_inode_hugepages+0x3db/0x3e2
> ....
> [  285.292389] Call Trace:
> [  285.321630]  hugetlbfs_evict_inode+0x1e/0x3e
> [  285.372707]  evict+0xdb/0x1af
> [  285.408185]  iput+0x1a2/0x1f7
> [  285.443661]  dentry_unlink_inode+0xc6/0xf0
> [  285.492661]  __dentry_kill+0xd8/0x18d
> [  285.536459]  dput+0x1b5/0x1ed
> [  285.571939]  __fput+0x18b/0x216
> [  285.609495]  ____fput+0xe/0x10
> [  285.646030]  task_work_run+0x90/0xa7
> [  285.688788]  exit_to_usermode_loop+0xdd/0x116
> [  285.740905]  do_syscall_64+0x187/0x1ae
> [  285.785740]  entry_SYSCALL_64_after_hwframe+0x150/0x0
> 
> Suggested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Signed-off-by: Jane Chu <jane.chu@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

with Cc: stable and Fixes: tag as suggested by Mike.

I also agree with Matthew that the comment should be associated with
hugetlb_vm_ops/shm_vm_ops.

Thanks!

> ---
>  include/linux/mm.h |  7 +++++++
>  ipc/shm.c          | 12 ++++++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a0fbb9ffe380..0c759379f2d9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -387,6 +387,13 @@ enum page_entry_size {
>   * These are the virtual MM functions - opening of an area, closing and
>   * unmapping it (needed to keep files on disk up-to-date etc), pointer
>   * to the functions called when a no-page or a wp-page exception occurs.
> + *
> + * Note, when a new function is introduced to vm_operations_struct and
> + * added to hugetlb_vm_ops, please consider adding the function to
> + * shm_vm_ops. This is because under System V memory model, though
> + * mappings created via shmget/shmat with "huge page" specified are
> + * backed by hugetlbfs files, their original vm_ops are overwritten with
> + * shm_vm_ops.
>   */
>  struct vm_operations_struct {
>  	void (*open)(struct vm_area_struct * area);
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 051a3e1fb8df..fefa00d310fb 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -427,6 +427,17 @@ static int shm_split(struct vm_area_struct *vma, unsigned long addr)
>  	return 0;
>  }
>  
> +static unsigned long shm_pagesize(struct vm_area_struct *vma)
> +{
> +	struct file *file = vma->vm_file;
> +	struct shm_file_data *sfd = shm_file_data(file);
> +
> +	if (sfd->vm_ops->pagesize)
> +		return sfd->vm_ops->pagesize(vma);
> +
> +	return PAGE_SIZE;
> +}
> +
>  #ifdef CONFIG_NUMA
>  static int shm_set_policy(struct vm_area_struct *vma, struct mempolicy *new)
>  {
> @@ -554,6 +565,7 @@ static const struct vm_operations_struct shm_vm_ops = {
>  	.close	= shm_close,	/* callback for when the vm-area is released */
>  	.fault	= shm_fault,
>  	.split	= shm_split,
> +	.pagesize = shm_pagesize,
>  #if defined(CONFIG_NUMA)
>  	.set_policy = shm_set_policy,
>  	.get_policy = shm_get_policy,
> -- 
> 2.15.GIT
> 

-- 
Michal Hocko
SUSE Labs