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

[Matthew Wilcox]

On Tue, Jun 22, 2021 at 11:28:30AM -0700, Linus Torvalds wrote:
> On Tue, Jun 22, 2021 at 11:23 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > It wouldn't be _that_ bad necessarily.  filemap_fault:
> 
> It's not actually the mm code that is the biggest problem. We
> obviously already have readahead support.
> 
> It's the *fault* side.
> 
> In particular, since the fault would return without actually filling
> in the page table entry (because the page isn't ready yet, and you
> cannot expose it to other threads!), you also have to jump over the
> instruction that caused this all.

Oh, I was assuming that it'd be a function call like
get_user_pages_fast(), not an instruction that was specially marked to
be jumped over.  Gag reflex diminishing now?