On Sun, 26 Jan 2025 at 09:02, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
>
> Hello there, a blast from the past.
>
> I see this has landed in b90197b655185a11640cce3a0a0bc5d8291b8ad2
Whee. What archeology are you doing to notice this decade-old issue?
> I came here from looking at a pwrite vs will-it-scale and noticing that
> pre-faulting eats CPU (over 5% on my Sapphire Rapids) due to SMAP trips.
Ugh. Yeah, turning SMAP on/off is expensive on most cores (apparently
fixed in AMD Zen 5).
> It used to be that pre-faulting was avoided specifically for that
> reason, but it got temporarily reverted due to bugs in ext4, to quote
> Linus (see 00a3d660cbac05af34cca149cb80fb611e916935):
Yeah, I think we should revert the revert (except we've done other
changes in the last decade - surprise surprise - so it would be a
completely manual revert).
If you send me a tested revert of the revert (aka re-do) of the "don't
pre-fault" patch, I'll apply it.
Note that the ext4 problem could exist in other filesystems, so we
might have to revert (again). It's not necessarily that ext4 was
_particularly_ buggy, it's quite possible that the problem was
originally found on ext4 just because it was more widely used than
others.
Linus
