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Message-ID: <2caf088c-e321-428e-afce-b1c11f52bc3f@lucifer.local>
Date: Mon, 20 Oct 2025 14:55:51 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
baohua@...nel.org, ioworker0@...il.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Wei Yang <richard.weiyang@...il.com>
Subject: Re: [PATCH mm-new v2 1/1] mm/khugepaged: guard is_zero_pfn() calls
with pte_present()
On Sat, Oct 18, 2025 at 12:33:33AM +0800, Lance Yang wrote:
>
>
> On 2025/10/17 23:44, Lorenzo Stoakes wrote:
> > On Fri, Oct 17, 2025 at 05:38:47PM +0800, Lance Yang wrote:
> > > From: Lance Yang <lance.yang@...ux.dev>
> > >
> > > A non-present entry, like a swap PTE, contains completely different data
> > > (swap type and offset). pte_pfn() doesn't know this, so if we feed it a
> > > non-present entry, it will spit out a junk PFN.
> >
> > It feels like this somewhat contradicts points I've made on the original series
> > re the is_swap_pte() stuff. Sigh.
>
> My bad. I didn't get your point before ...
Don't worry, this is a problem that existed already and needs addressing, series
incoming :)
>
> And this patch is not intended to touch is_swap_pte() ...
Ack
>
> >
> > I guess that's _such a mess_ it's hard to avoid though.
> >
> > And I guess it's reasonable that !pte_present() means we can't expect a valid
> > PFN though.
>
> Yes, I think we expect a valid PFN must be under pte_present().
Yes
>
> >
> > >
> > > What if that junk PFN happens to match the zeropage's PFN by sheer
> > > chance? While really unlikely, this would be really bad if it did.
> > >
> > > So, let's fix this potential bug by ensuring all calls to is_zero_pfn()
> > > in khugepaged.c are properly guarded by a pte_present() check.
> > >
> > > Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> >
> > Not sure I really suggested something that strictly contradicts points I
> > made... but I guess I did suggest guarding this stuff more carefully.
>
> Sorry, I didn't catch you again ... Will drop the Suggested-by tag.
Nah it's fine sorry, I think in general you are doing what I asked.
I'm going to address the is_swap_pte() stuff separately anyway :) have discussed
with David off-list a lot. Think I have a sensible plan...
>
> >
> > > Reviewed-by: Dev Jain <dev.jain@....com>
> > > Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> > > Reviewed-by: Wei Yang <richard.weiyang@...il.com>
> > > Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> > > ---
> > > Applies against commit 0f22abd9096e in mm-new.
> > >
> > > v1 -> v2:
> > > - Collect Reviewed-by from Dev, Wei and Baolin - thanks!
> > > - Reduce a level of indentation (per Dev)
> > > - https://lore.kernel.org/linux-mm/20251016033643.10848-1-lance.yang@linux.dev/
> > >
> > > mm/khugepaged.c | 29 ++++++++++++++++-------------
> > > 1 file changed, 16 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index d635d821f611..648d9335de00 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -516,7 +516,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
> > > pte_t pteval = ptep_get(_pte);
> > > unsigned long pfn;
> > >
> > > - if (pte_none(pteval))
> > > + if (!pte_present(pteval))
> > > continue;
> > > pfn = pte_pfn(pteval);
> > > if (is_zero_pfn(pfn))
> > > @@ -690,17 +690,18 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> > > address += nr_ptes * PAGE_SIZE) {
> > > nr_ptes = 1;
> > > pteval = ptep_get(_pte);
> > > - if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > > + if (pte_none(pteval) ||
> > > + (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) {
> > > add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> > > - if (is_zero_pfn(pte_pfn(pteval))) {
> > > - /*
> > > - * ptl mostly unnecessary.
> > > - */
> > > - spin_lock(ptl);
> > > - ptep_clear(vma->vm_mm, address, _pte);
> > > - spin_unlock(ptl);
> > > - ksm_might_unmap_zero_page(vma->vm_mm, pteval);
> > > - }
> > > + if (pte_none(pteval))
> > > + continue;
> >
> > Yeah I'm not sure I really love this refactoring.
> >
> > Can be:
> >
> > if (!is_swap_pte(pteval)) {
> > add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> > if (!is_zero_pfn(pte_pfn(pteval)))
> > continue;
> >
> > ...
> > }
> >
> > Doing pte_pfn() on a pte_none() PTE is fine.
> >
> > Obviously as theree's a lot of hate for is_swap_pte() you could also do:
> >
> > if (pte_none(pteval) || pte_present(pteval)) {
> > ...
> > }
> >
> > Which literally open-codes !is_swap_pte().
> >
> > At the same time, this makes very clear that PTE none is OK.
>
> Emm, I'd prefer the new helper pte_none_or_zero() here:
>
> if (pte_none_or_zero(pteval)) {
> add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> if (pte_none(pteval))
> continue;
> ....
> }
> That looks really clean and simple for me ;)
Haha yeah sure that's better :)
>
> >
> > > + /*
> > > + * ptl mostly unnecessary.
> > > + */
> > > + spin_lock(ptl);
> > > + ptep_clear(vma->vm_mm, address, _pte);
> > > + spin_unlock(ptl);
> > > + ksm_might_unmap_zero_page(vma->vm_mm, pteval);
> > > } else {
> > > struct page *src_page = pte_page(pteval);
> > >
> > > @@ -794,7 +795,8 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
> > > unsigned long src_addr = address + i * PAGE_SIZE;
> > > struct page *src_page;
> > >
> > > - if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > > + if (pte_none(pteval) ||
> > > + (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) {
> > > clear_user_highpage(page, src_addr);
> > > continue;
> > > }
> > > @@ -1294,7 +1296,8 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> > > goto out_unmap;
> > > }
> > > }
> > > - if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > > + if (pte_none(pteval) ||
> > > + (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) {
> > > ++none_or_zero;
> > > if (!userfaultfd_armed(vma) &&
> > > (!cc->is_khugepaged ||
> > > --
> > > 2.49.0
> > >
> >
> > I mean all of this seems super gross anyway. We're constantly open-coding the
> > same check over and over again.
> >
> > static inline bool pte_is_none_or_zero(pte_t pteval)
> > {
> > if (is_swap_pte(pteval))
> > return false;
> >
> > return is_zero_pfn(pte_pfn(pteval));
> > }
> >
> > Put somewhere in a relevant header file.
> >
> > Or again, if there's distaste at is_swap_pte(), and here maybe it's more valid
> > not to use it (given name of function).
> >
> > static inline bool pte_is_none_or_zero(pte_t pteval)
> > {
> > /* Non-present entries do not have a PFN to check. */
> > if (!pte_present(pteval))
> > return false;
> >
> > if (pte_none(pteval))
> > return true;
> >
> > return is_zero_pfn(pte_pfn(pteval));
> > }
>
> Yeah, I'll put pte_none_or_zero() in this file first.
>
> static inline bool pte_none_or_zero(pte_t pte)
> {
> if (pte_none(pte))
> return true;
> return pte_present(pte) && is_zero_pfn(pte_pfn(pte));
> }
Well I intended this to be in some general header file, but it's not obvious
actually where would make sense so feel free to put here as a static (no need
for inline).
>
> >
> > I think I'm going to do a series to addres the is_swap_pte() mess actually, as
> > this whole thing is very frustrating.
>
> Excellent! Looking forward to your series to clean that up ;)
Already started on it :)
>
> Thanks,
> Lance
Cheers, Lorenzo
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