Re: [PATCH v2 4/9] mm/mshare: Add a read operation for msharefs files

Khalid Aziz
2022-06-30T22:28:51Z

On 6/30/22 15:27, Darrick J. Wong wrote:
> On Wed, Jun 29, 2022 at 04:53:55PM -0600, Khalid Aziz wrote:
>> When a new file is created under msharefs, allocate a new mm_struct
>> that will hold the VMAs for mshare region. Also allocate structure
>> to defines the mshare region and add a read operation to the file
>> that returns this information about the mshare region. Currently
>> this information is returned as a struct:
>>
>> struct mshare_info {
>> unsigned long start;
>> unsigned long size;
>> };
>>
>> This gives the start address for mshare region and its size.
>>
>> Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
>> ---
>> include/uapi/linux/mman.h | 5 +++
>> mm/mshare.c | 64 ++++++++++++++++++++++++++++++++++++++-
>> 2 files changed, 68 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
>> index f55bc680b5b0..56fe446e24b1 100644
>> --- a/include/uapi/linux/mman.h
>> +++ b/include/uapi/linux/mman.h
>> @@ -41,4 +41,9 @@
>> #define MAP_HUGE_2GB HUGETLB_FLAG_ENCODE_2GB
>> #define MAP_HUGE_16GB HUGETLB_FLAG_ENCODE_16GB
>>
>> +struct mshare_info {
>> + unsigned long start;
>> + unsigned long size;
>
> You might want to make these explicitly u64, since this is userspace
> ABI and you never know when someone will want to do something crazy like
> run 32-bit programs with mshare files.
>
> Also you might want to add some padding fields for flags, future
> expansion, etc.

That sounds like a good idea. I will queue it up for next version of patch series.

>
>> +};
>> +
>> #endif /* _UAPI_LINUX_MMAN_H */
>> diff --git a/mm/mshare.c b/mm/mshare.c
>> index 2d5924d39221..d238b68b0576 100644
>> --- a/mm/mshare.c
>> +++ b/mm/mshare.c
>> @@ -22,8 +22,14 @@
>> #include <uapi/linux/magic.h>
>> #include <uapi/linux/limits.h>
>> #include <uapi/linux/mman.h>
>> +#include <linux/sched/mm.h>
>>
>> static struct super_block *msharefs_sb;
>> +struct mshare_data {
>> + struct mm_struct *mm;
>> + refcount_t refcnt;
>> + struct mshare_info *minfo;
>> +};
>>
>> static const struct inode_operations msharefs_dir_inode_ops;
>> static const struct inode_operations msharefs_file_inode_ops;
>> @@ -34,8 +40,29 @@ msharefs_open(struct inode *inode, struct file *file)
>> return simple_open(inode, file);
>> }
>>
>> +static ssize_t
>> +msharefs_read(struct kiocb *iocb, struct iov_iter *iov)
>> +{
>> + struct mshare_data *info = iocb->ki_filp->private_data;
>> + size_t ret;
>> + struct mshare_info m_info;
>> +
>> + if (info->minfo != NULL) {
>> + m_info.start = info->minfo->start;
>> + m_info.size = info->minfo->size;
>> + } else {
>> + m_info.start = 0;
>> + m_info.size = 0;
>
> Hmmm, read()ing out the shared mapping information. Heh.
>
> When does this case happen? Is it before anybody mmaps this file into
> an address space?
>

It can happen before or after the first mmap which will establish the start address and size. Hence I have to account
for both cases.

>> + }
>> + ret = copy_to_iter(&m_info, sizeof(m_info), iov);
>> + if (!ret)
>> + return -EFAULT;
>> + return ret;
>> +}
>> +
>> static const struct file_operations msharefs_file_operations = {
>> .open = msharefs_open,
>> + .read_iter = msharefs_read,
>> .llseek = no_llseek,
>> };
>>
>> @@ -73,12 +100,43 @@ static struct dentry
>> return ERR_PTR(-ENOMEM);
>> }
>>
>> +static int
>> +msharefs_fill_mm(struct inode *inode)
>> +{
>> + struct mm_struct *mm;
>> + struct mshare_data *info = NULL;
>> + int retval = 0;
>> +
>> + mm = mm_alloc();
>> + if (!mm) {
>> + retval = -ENOMEM;
>> + goto err_free;
>> + }
>> +
>> + info = kzalloc(sizeof(*info), GFP_KERNEL);
>> + if (!info) {
>> + retval = -ENOMEM;
>> + goto err_free;
>> + }
>> + info->mm = mm;
>> + info->minfo = NULL;
>> + refcount_set(&info->refcnt, 1);
>> + inode->i_private = info;
>> +
>> + return 0;
>> +
>> +err_free:
>> + if (mm)
>> + mmput(mm);
>> + kfree(info);
>> + return retval;
>> +}
>> +
>> static struct inode
>> *msharefs_get_inode(struct super_block *sb, const struct inode *dir,
>> umode_t mode)
>> {
>> struct inode *inode = new_inode(sb);
>> -
>> if (inode) {
>> inode->i_ino = get_next_ino();
>> inode_init_owner(&init_user_ns, inode, dir, mode);
>> @@ -89,6 +147,10 @@ static struct inode
>> case S_IFREG:
>> inode->i_op = &msharefs_file_inode_ops;
>> inode->i_fop = &msharefs_file_operations;
>> + if (msharefs_fill_mm(inode) != 0) {
>> + discard_new_inode(inode);
>> + inode = ERR_PTR(-ENOMEM);
>
> Is it intentional to clobber the msharefs_fill_mm return value and
> replace it with ENOMEM?

ENOMEM sounded like the right value to return from msharefs_get_inode() in case of failure. On the other hand, there
isn't much of a reason to not just return the return value from msharefs_fill_mm(). I can change that.

Thanks for the review.

--
Khalid