Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin_pages*() routines

Christoph Hellwig
2022-09-07T08:45:47Z

On Tue, Sep 06, 2022 at 12:21:06PM +0200, Jan Kara wrote:
> > For FOLL_PIN callers, never pin bvec and kvec pages: For file systems
> > not acquiring a reference is obviously safe, and the other callers will
> > need an audit, but I can't think of why it woul ever be unsafe.
>
> Are you sure about "For file systems not acquiring a reference is obviously
> safe"? I can see places e.g. in orangefs, afs, etc. which create bvec iters
> from pagecache pages. And then we have iter_file_splice_write() which
> creates bvec from pipe pages (which can also be pagecache pages if
> vmsplice() is used). So perhaps there are no lifetime issues even without
> acquiring a reference (but looking at the code I would not say it is
> obvious) but I definitely don't see how it would be safe to not get a pin
> to signal to filesystem backing the pagecache page that there is DMA
> happening to/from the page.

I mean in the context of iov_iter_get_pages callers, that is direct
I/O. Direct callers of iov_iter_bvec which then pass that iov to
->read_iter / ->write_iter will need to hold references (those are
the references that the callers of iov_iter_get_pages rely on!).