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

On Tue, Jun 22, 2021 at 11:28:30AM -0700, Linus Torvalds wrote:

On Tue, Jun 22, 2021 at 11:23 AM Matthew Wilcox [off-list ref] wrote:

quoted

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?