Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"?

On Jun 22, 2021, at 11:07 AM, Linus Torvalds [off-list ref] wrote:

On Tue, Jun 22, 2021 at 11:05 AM Matthew Wilcox [off-list ref] wrote:

quoted

Huh?  Last I checked, the fault_in_readable actually read a byte from
the page.  It has to wait for the read to complete before that can
happen.

Yeah, we don't have any kind of async fault-in model.

I'm not sure how that would even look. I don't think it would
necessarily be *impossible* (special marker in the exception table to
let the fault code know that this is a "prepare" fault), but it would
be pretty challenging.

I did send an RFC some time ago for “prepare” fault, but it the RFC
requires some more work.

https://lore.kernel.org/lkml/20210225072910.2811795-1-namit@vmware.com/ (local)