Re: [RFC PATCH] tmpfs: support user quotas

Davidlohr Bueso
2011-11-07T12:47:03Z

On Mon, 2011-11-07 at 04:11 -0500, Valdis.Kletnieks@vt.edu wrote:
> On Sun, 06 Nov 2011 18:15:01 -0300, Davidlohr Bueso said:
>
> > @@ -1159,7 +1159,12 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> > struct page **pagep, void **fsdata)
>
> > + if (atomic_long_read(&user->shmem_bytes) + len >
> > + rlimit(RLIMIT_TMPFSQUOTA))
> > + return -ENOSPC;
>
> Is this a per-process or per-user limit? If it's per-process, it doesn't
> really do much good, because a user can use multiple processes to over-run the
> limit (either intentionally or accidentally).

This is a per-user limit.
>
> > @@ -1169,10 +1174,12 @@ shmem_write_end(struct file *file, struct address_space *mapping,
> > struct page *page, void *fsdata)
>
> > + if (pos + copied > inode->i_size) {
> > i_size_write(inode, pos + copied);
> > + atomic_long_add(copied, &user->shmem_bytes);
> > + }
> If this is per-user, it's racy with shmem_write_begin() - two processes can hit
> the write_begin(), be under quota by (say) 1M, but by the time they both
> complete the user is 1M over the quota.
>
I guess using a spinlock instead of atomic operations would serve the
purpose.

> > @@ -1535,12 +1542,15 @@ static int shmem_unlink(struct inode *dir, struct dentry *dentry)
> > + struct user_struct *user = current_user();
> > + atomic_long_sub(inode->i_size, &user->shmem_bytes);
>
> What happens here if user 'fred' creates a file on a tmpfs, and then logs out so he has
> no processes running, and then root does a 'find tmpfs -user fred -exec rm {} \;' to clean up?
> We just decremented root's quota, not fred's....
>
Would the same would occur with mqueues? I haven't tested it but I don't
see anywhere that user->mq_bytes is decreased like this.

Thanks,
Davidlohr