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!).
