Re: [PATCH v7 18/19] iov_iter: Introduce nofault flag to disable page faults

[Andreas Gruenbacher]

On Fri, Aug 27, 2021 at 8:47 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Aug 27, 2021 at 06:49:25PM +0200, Andreas Gruenbacher wrote:
> > Introduce a new nofault flag to indicate to get_user_pages to use the
> > FOLL_NOFAULT flag.  This will cause get_user_pages to fail when it
> > would otherwise fault in a page.
> >
> > Currently, the noio flag is only checked in iov_iter_get_pages and
> > iov_iter_get_pages_alloc.  This is enough for iomaop_dio_rw, but it
> > may make sense to check in other contexts as well.
>
> I can live with that, but
>         * direct assignments (as in the next patch) are fucking hard to
> grep for.  Is it intended to be "we set it for duration of primitive",
> or...?

It's for this kind of pattern:

       pagefault_disable();
       to->nofault = true;
       ret = iomap_dio_rw(iocb, to, &gfs2_iomap_ops, NULL,
                          IOMAP_DIO_PARTIAL, written);
       to->nofault = false;
       pagefault_enable();

Clearing the flag at the end isn't strictly necessary, but it kind of
documents that the flag pertains to iomap_dio_rw and not something
else.

>         * it would be nice to have a description of intended semantics
> for that thing.  This "may make sense to check in other contexts" really
> needs to be elaborated (and agreed) upon.  Details, please.

Maybe the description should just be something like:

"Introduce a new nofault flag to indicate to iov_iter_get_pages not to
fault in user pages.

This is implemented by passing the FOLL_NOFAULT flag to get_user_pages,
which causes get_user_pages to fail when it would otherwise fault in a
page. We'll use the ->nofault flag to prevent iomap_dio_rw from faulting
in pages when page faults are not allowed."

Thanks,
Andreas