Messages in this thread | | | From | Andreas Gruenbacher <> | Date | Fri, 27 Aug 2021 21:56:10 +0200 | Subject | Re: [PATCH v7 18/19] iov_iter: Introduce nofault flag to disable page faults |
| |
On Fri, Aug 27, 2021 at 8:47 PM Al Viro <viro@zeniv.linux.org.uk> 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
| |