On Fri, Aug 09, 2024 at 02:44:21PM -0400, Josef Bacik wrote:
> FS_PRE_ACCESS or FS_PRE_MODIFY will be generated on page fault depending
> on the faulting method.
>
> This pre-content event is meant to be used by hierarchical storage
> managers that want to fill in the file content on first read access.
>
> Export a simple helper that file systems that have their own ->fault()
> will use, and have a more complicated helper to be do fancy things with
> in filemap_fault.
>
> Signed-off-by: Josef Bacik <[email protected]>
....
> +/**
> + * filemap_maybe_emit_fsnotify_event - maybe emit a pre-content event.
> + * @vmf: struct vm_fault containing details of the fault.
> + *
> + * If we have a pre-content watch on this file we will emit an event for this
> + * range. If we return anything the fault caller should return immediately,
> we
> + * will return VM_FAULT_RETRY if we had to emit an event, which will trigger
> the
> + * fault again and then the fault handler will run the second time through.
> + *
> + * Return: a bitwise-OR of %VM_FAULT_ codes, 0 if nothing happened.
> + */
> +vm_fault_t filemap_maybe_emit_fsnotify_event(struct vm_fault *vmf)
> +{
> + struct file *fpin = NULL;
> + vm_fault_t ret;
> +
> + ret = __filemap_maybe_emit_fsnotify_event(vmf, &fpin);
> + if (ret) {
> + if (fpin)
> + fput(fpin);
> + return ret;
> + } else if (fpin) {
> + fput(fpin);
> + return VM_FAULT_RETRY;
> + }
Logic is back to front. Both paths have to check for fpin,
only one fpin path needs to modify ret:
ret = __filemap_maybe_emit_fsnotify_event(vmf, &fpin);
if (fpin) {
fput(fpin);
if (!ret)
ret = VM_FAULT_RETRY;
}
return ret;
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(filemap_maybe_emit_fsnotify_event);
> +
> /**
> * filemap_fault - read in file data for page fault handling
> * @vmf: struct vm_fault containing details of the fault
> @@ -3299,6 +3391,19 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
> if (unlikely(index >= max_idx))
> return VM_FAULT_SIGBUS;
>
> + /*
> + * If we have pre-content watchers then we need to generate events on
> + * page fault so that we can populate any data before the fault.
> + */
> + ret = __filemap_maybe_emit_fsnotify_event(vmf, &fpin);
> + if (unlikely(ret)) {
> + if (fpin) {
> + fput(fpin);
> + ret |= VM_FAULT_RETRY;
> + }
> + return ret;
> + }
Why do we or in VM_FAULT_RETRY here where as the previous case we
simply return VM_FAULT_RETRY? Which one of these is wrong? If both
are correct, then a comment explaining this is in order...
-Dave.
--
Dave Chinner
[email protected]
