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    SubjectRe: [RFC v10][PATCH 08/13] Dump open file descriptors
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    On Fri, 2008-11-28 at 10:19 +0000, Al Viro wrote:
    > On Wed, Nov 26, 2008 at 08:04:39PM -0500, Oren Laadan wrote:
    > > +int cr_scan_fds(struct files_struct *files, int **fdtable)
    > > +{
    > > + struct fdtable *fdt;
    > > + int *fds;
    > > + int i, n = 0;
    > > + int tot = CR_DEFAULT_FDTABLE;
    > > +
    > > + fds = kmalloc(tot * sizeof(*fds), GFP_KERNEL);
    > > + if (!fds)
    > > + return -ENOMEM;
    > > +
    > > + /*
    > > + * We assume that the target task is frozen (or that we checkpoint
    > > + * ourselves), so we can safely proceed after krealloc() from where
    > > + * we left off; in the worst cases restart will fail.
    > > + */
    >
    > Task may be frozen, but it may share the table with any number of other
    > tasks...

    First of all, thanks for looking at this, Al.

    I think Oren's assumption here is that all tasks possibly sharing the
    table would be frozen. I don't think that's a good assumption,
    either. :)

    This would be a lot safer and bulletproof if we size the allocation
    ahead of time, take all the locks, then retry if the size has changed.

    I think that will just plain work of we do this:

    > > + spin_lock(&files->file_lock);
    > > + rcu_read_lock();
    > > + fdt = files_fdtable(files);
    > > + for (i = 0; i < fdt->max_fds; i++) {
    > > + if (!fcheck_files(files, i))
    > > + continue;
    > > + if (n == tot) {
    > > + /*
    > > + * fcheck_files() is safe with drop/re-acquire
    > > + * of the lock, because it tests: fd < max_fds
    > > + */
    > > + spin_unlock(&files->file_lock);
    > > + rcu_read_unlock();
    > > + tot *= 2; /* won't overflow: kmalloc will fail */

    free(fds);
    goto first_kmalloc_in_this_function;

    > > + }
    > > + fds[n++] = i;
    > > + }
    > > + rcu_read_unlock();
    > > + spin_unlock(&files->file_lock);
    > > +
    > > + *fdtable = fds;
    > > + return n;
    > > +}

    Right?

    > > + switch (inode->i_mode & S_IFMT) {
    > > + case S_IFREG:
    > > + fd_type = CR_FD_FILE;
    > > + break;
    > > + case S_IFDIR:
    > > + fd_type = CR_FD_DIR;
    > > + break;
    > > + case S_IFLNK:
    > > + fd_type = CR_FD_LINK;
    >
    > Opened symlinks? May I have whatever you'd been smoking, please?

    Ugh, that certainly doesn't have any place here. I do wonder if Oren
    had some use for that in the fully put together code, but it can
    certainly go for now.

    I'll send patches for these shortly.

    -- Dave



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