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
