Re: [PATCH 1/2] mm/khugepaged: fix spurious -EINVAL from sub-PMD MADV_COLLAPSE range

[Lorenzo Stoakes <ljs@kernel.org>]

On Fri, May 08, 2026 at 04:02:37PM +0100, Lorenzo Stoakes wrote:
> On Fri, May 08, 2026 at 02:27:37PM +0200, David Hildenbrand (Arm) wrote:
> > On 5/7/26 09:05, Chen Wandun wrote:
> > > madvise_collapse() computes the THP-aligned window:
> > >
> > >   hstart = (start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK  /* round up  */
> > >   hend   =  end   &  HPAGE_PMD_MASK                    /* round down */
> > >
> > > Previously this was done after kmalloc_obj(), so problem arose when
> > > the range contained no complete PMD-aligned window (hstart >= hend).
> > >
> > > When hstart > hend, (hend - hstart) wraps unsigned to a huge value, the
> > > final comparison fails and -EINVAL is returned instead of 0.  Consider
>
> I think both should return -EINVAL.

Correction: I changed my mind (see below), and think == should return 0 simply
for compatibility reasons. Though honestly both really should have been -EINVAL
from the start...

>
> > > two single-page calls on a 2 MiB-aligned address:
> > >
> > >     /* hstart == hend == aligned  ->  0 == 0  ->  returns 0 */
> > >     madvise(aligned, PAGE_SIZE, MADV_COLLAPSE);
>
> What's aligned? You're putting a random variable name in there? Presumably a PMD-aligned address?
>
> > >
> > >     /* hstart = aligned + 2MiB, hend = aligned
> > >      * (hend - hstart) wraps unsigned  ->  returns -EINVAL */
> > >     madvise(aligned + PAGE_SIZE, PAGE_SIZE, MADV_COLLAPSE);
> > >
> > > Both calls cover less than one THP and collapse nothing; both should
> > > return 0.
>
> Disagree.
>
> >
> > Okay, so we talk about a "userspace is being stupid" scenario.
>
> Yes!
>
> I feel that -EINVAL is correct for hend > hstart, and I think it might even be a
> userland A[BP]I break to change it (maybe somebody, somewhere is being foolish
> enough to use this to also validate input ranges).
>
> The weirdness is when hstart == hend being 0 but that's sort of established
> behaviour I guess.
>
> >
> > >
> > > In addition, kmalloc_obj(), mmgrab() and lru_add_drain_all() were all
> > > called before discovering there was nothing to do, only for the code
> > > to kfree() and return immediately after.
> >
> > Just a comment as you motivate here why this is suboptimal: we do not care about
> > a "userspace is being stupid" scenario being fast.
>
> Yes, in general - so what? The user is doing stupid things, so the user wins
> stupid prizes?
>
> >
> > >
> > > Fix both by computing hstart/hend after thp_vma_allowable_order() but
> > > before kmalloc_obj(), and returning 0 early when hstart >= hend.
> > >
> > > Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
> >
> > Fixes: is likely ok, but I don't think we want to treat this as a hotfix or CC
> > stable.
>
> I'm not sure I want a fixes here, this isn't really fixing anything. This isn't
> a bug afaik, it's just us not handling this brilliantly, but (possibly by
> mistake) getting the right output.
>
> >
> > > Signed-off-by: Chen Wandun <chenwandun@lixiang.com>
>
> I put this patch through AI detection and it's telling me there's an 80% chance
> this whole thing is LLM-generated, which is making me grumpy.
>
> Can you confirm that this is, in fact, your own work? Plagiarism is not a nice
> thing to do, and THP doesn't need more traffic, we're overloaded as it is.
>
> > > ---
> > >  mm/khugepaged.c | 9 ++++++---
> > >  1 file changed, 6 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index b8452dbdb043..92473d93e837 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -2836,6 +2836,12 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> > >  	if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_FORCED_COLLAPSE, PMD_ORDER))
> > >  		return -EINVAL;
> > >
> > > +	hstart = (start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> > > +	hend = end & HPAGE_PMD_MASK;
>
> See below re: conflict.
>
> > > +
> > > +	if (hstart >= hend)
> > > +		return 0;
>
> 	if (hstart > hend)
> 		return -EINVAL;
> 	/* For compatibility, users may rely on this. */
> 	if (hstart == hend)
> 		return 0;
>
> Is probably better.
>
> But I'm not sure what the point is if we're already doing this behaviour?
>
> > > +
> > >  	cc = kmalloc_obj(*cc);
> > >  	if (!cc)
> > >  		return -ENOMEM;
> > > @@ -2845,9 +2851,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> > >  	mmgrab(mm);
> > >  	lru_add_drain_all();
> > >
> > > -	hstart = (start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> > > -	hend = end & HPAGE_PMD_MASK;
> > > -
> > >  	for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
> > >  		enum scan_result result = SCAN_FAIL;
> > >
> >
> > In general, LGTM, but see for conflict:
> > https://lore.kernel.org/all/20260409014323.2385982-1-ye.liu@linux.dev/
>
> Please use mm-unstable as a basis for your mm work Chen, this is something you
> need to fix, the patch above has been around for a while and is in
> mm-unstable.
>
> You have patches in mm already so you should know better by now.
>
> But I'm really not sure I'm in favour of this anyway. I'll defer to David but
> this feels useless to me.
>
> >
> >
> > --
> > Cheers,
> >
> > David
>
> Thanks, Lorenzo