Re: [PATCH] mm/madvise: reclaim isolated folios if PTE restart fails

[Lorenzo Stoakes (ARM)]

On Tue, Sep 15, 2026 at 01:00:31PM -0400, Gregory Price wrote:
> On Tue, Sep 15, 2026 at 04:55:47PM +0100, Lorenzo Stoakes (ARM) wrote:
> > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > index 574aa2bb7c7e..ba5a3d77241a 100644
> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -464,7 +464,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >
> > This function is truly some of the absolute worst code I've read in
> > mm. Shocking.
> >
>
> relevant meme: https://i.imgur.com/W0N40BK.png

Content not viewable in my region :'( seems I do not have a loicense for
that!

>
> No argument here. This function in particular is really horrendous.
>
> I think i'm now 4 or 5 stacks deep on the "Thanks for fixing a bug, but
> while reviewing your fix sashiko found yet another CVE-worthy bug".

Yeah, I WISH it wouldn't do that.

I mean it's helpful to some degree and it finds real stuff.

But it's _where_ it does that and the workload++ aspect of it.

>
> I'm convinced (like you below) madvise needs some attention.
>
> > >  restart:
> > >  	start_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> > >  	if (!start_pte)
> > > -		return 0;
> > > +		goto out;
> > >  	flush_tlb_batched_pending(mm);
> > >  	lazy_mmu_mode_enable();
> > >  	for (; addr < end; pte += nr, addr += nr * PAGE_SIZE) {
> > > @@ -568,6 +568,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> > >  			folio_deactivate(folio);
> > >  	}
> > >
> > > +out:
> >
> > Seems odd to put this here for a condition that explicitly guarantees
> > !start_pte? It should be before the if (pageout) reclaim_pages(...);
> > surely?
> >
>
> I wanted to prevent any future use of the label from skipping the
> start_pte check.  The function is so bad I was err'ing on the side
> of being a bit defensive without making it worse.
>
> I suppose reasonable to say the goto is worse.

Yeah I know it's all a bit much of a muchness, and obviously my suggestion
_duplicates code_ which is also not wonderful.

But the control flow in the function I think is a bigger issue.

>
> > I honestly wonder whether, rather than adding yet another label/goto into this
> > absolute bloody mess, whether we should just live with a bit of duplication and do:
> >
> > 	if (!start_pte) {
> > 		if (pageout)
> > 			reclaim_pages(&folio_list);
> > 		return 0;
> > 	}
> >
> > I actually think that'd be clearer at this point than throwing in some more
> > indirection.
> >
>
> I'll spin a v2.

Thanks!

>
> > That's for the backport but somebody needs to rework this entire bloody
> > function going forwards...
>
> The longer I look at madvise, the more I think it is a wart on mm/
> as-written.  A userland-driven page table walker that requires all
> the special-casing of Yooj Pages, zone device, swap, soft leafs, a
> bunch of tlb flushing 5-tabs deep...
>
> Scary surface.
>
> I've been trying to think about how we might make this more reasonable,
> but generalizing the different walkers across mm is pretty hard.

I mean it's a vital interface, but it often feels the wrong one (it's meant
to be advice but several cases cannot actually be 'advisory'), and it's all
a bit of a mess.

I wouldn't be against a new interface (new system calls aren't a big deal)
but it'd have to be carefully thought through.

Like say, the complete diametric opposite of e.g. prctl :)

Anyway we have to keep supporting what's there indefinitely because users
and such and in any case we can certainly improve hellish code in
madvise.c.

>
> ~Gregory

--
Cheers, Lorenzo