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SubjectRe: [RFC][PATCH 8/8] check files for checkpointability
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On Fri, 2009-02-27 at 18:57 -0800, Sukadev Bhattiprolu wrote: 
> Dave Hansen [dave@linux.vnet.ibm.com] wrote:
> |
> | Introduce a files_struct counter to indicate whether a particular
> | file_struct has ever contained a file which can not be
> | checkpointed. This flag is a one-way trip; once it is set, it may
> | not be unset.
> |
> | We assume at allocation that a new files_struct is clean and may
> | be checkpointed. However, as soon as it has had its files filled
> | from its parent's, we check it for real in __scan_files_for_cr().
> | At that point, we mark it if it contained any uncheckpointable
> | files.
>
> Hmm. Why not just copy ->may_checkpoint setting from parent (or old)
> files_struct ? If parent is not checkpointable, then child won't be
> and vice-versa - no ?

Because init does things that are uncheckpointable. If we purely
inherit, we'll never be able to checkpoint.

> | +static void __scan_files_for_cr(struct files_struct *files)
> | +{
> | + int i;
> | +
> | + for (i = 0; i < files->fdtab.max_fds; i++) {
> | + struct file *f = fcheck_files(files, i);
> | + if (!f)
> | + continue;
> | + if (cr_file_supported(f))
> | + continue;
> | + files_deny_checkpointing(files);
> | + }
> | +}
> | +
>
> A version of __scan_files_for_cr() for CONFIG_CHECKPOINT_RESTART=n or...
>
> | /*
> | * Allocate a new files structure and copy contents from the
> | * passed in files structure.
> | @@ -303,6 +318,9 @@ struct files_struct *dup_fd(struct files
> | goto out;
> |
> | atomic_set(&newf->count, 1);
> | +#ifdef CONFIG_CHECKPOINT_RESTART
> | + newf->may_checkpoint = 1;
> | +#endif
> |
> | spin_lock_init(&newf->file_lock);
> | newf->next_fd = 0;
> | @@ -396,6 +414,7 @@ struct files_struct *dup_fd(struct files
> |
> | rcu_assign_pointer(newf->fdt, new_fdt);
> |
> | + __scan_files_for_cr(newf);
>
> ... #ifdef CONFIG_CHECKPOINT_RESTART here ?

gcc isn't quite smart enough to figure out that this is a noop. Please
see my new set for a new compiler helper to solve this problem.

-- Dave



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