Re: [PATCH v2 0/4] Avoid live-lock in fault-in+uaccess loops with sub-page faults

[Andreas Gruenbacher]

On Fri, Dec 3, 2021 at 6:58 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Dec 3, 2021 at 7:29 AM Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote:
> > We're trying pretty hard to handle large I/O requests efficiently at
> > the filesystem level. A small, static upper limit in the fault-in
> > functions has the potential to ruin those efforts. So I'm not a fan of
> > that.
>
> I don't think fault-in should happen under any sane normal circumstances.
>
> Except for low-memory situations, and then you don't want to fault in
> large areas.
>
> Do you really expect to write big areas that the user has never even
> touched? That would be literally insane.
>
> And if the user _has_ touched them, then they'll in in-core. Except
> for the "swapped out" case.
>
> End result: this is purely a correctness issue, not a performance issue.

It happens when you mmap a file and write the mmapped region to
another file, for example. I don't think we want to make filesystems
go bonkers in such scenarios. Scaling down in response to memory
pressure sounds perfectly fine though.

Thanks,
Andreas