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    SubjectRe: [RFC PATCH] tmpfs: support user quotas
    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



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