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Message-ID: <b5627e83-489c-4e16-910c-fe7e56912793@linux.dev>
Date: Sat, 18 Oct 2025 00:33:33 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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 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 ...
And this patch is not intended to touch is_swap_pte() ...
>
> 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().
>
>>
>> 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.
>
>> 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 ;)
>
>> + /*
>> + * 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));
}
>
> 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 ;)
Thanks,
Lance
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