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

Jan Kara
2022-09-06T10:21:21Z

On Tue 06-09-22 00:48:49, Christoph Hellwig wrote:
> On Tue, Sep 06, 2022 at 12:44:28AM -0700, John Hubbard wrote:
> > OK, that part is clear.
> >
> > > - for the pin case don't use the existing bvec helper at all, but
> > > copy the logic for the block layer for not pinning.
> >
> > I'm almost, but not quite sure I get the idea above. Overall, what
> > happens to bvec pages? Leave the get_page() pin in place for FOLL_GET
> > (or USE_FOLL_GET), I suppose, but do...what, for FOLL_PIN callers?
>
> Do not change anyhing for FOLL_GET callers, as they are on the way out
> anyway.
>
> 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.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR