Re: [PATCH v3 03/16] mm: avoid unnecessary uses of is_swap_pte()

Zi Yan
2025-11-12T16:03:38Z

On 12 Nov 2025, at 10:59, Lorenzo Stoakes wrote:

> On Tue, Nov 11, 2025 at 09:58:36PM -0500, Zi Yan wrote:
>> On 10 Nov 2025, at 17:21, Lorenzo Stoakes wrote:
>>
>>> There's an established convention in the kernel that we treat PTEs as
>>> containing swap entries (and the unfortunately named non-swap swap entries)
>>> should they be neither empty (i.e. pte_none() evaluating true) nor present
>>> (i.e. pte_present() evaluating true).
>>>
>>> However, there is some inconsistency in how this is applied, as we also
>>> have the is_swap_pte() helper which explicitly performs this check:
>>>
>>> /* check whether a pte points to a swap entry */
>>> static inline int is_swap_pte(pte_t pte)
>>> {
>>> return !pte_none(pte) && !pte_present(pte);
>>> }
>>>
>>> As this represents a predicate, and it's logical to assume that in order to
>>> establish that a PTE entry can correctly be manipulated as a swap/non-swap
>>> entry, this predicate seems as if it must first be checked.
>>>
>>> But we instead, we far more often utilise the established convention of
>>> checking pte_none() / pte_present() before operating on entries as if they
>>> were swap/non-swap.
>>>
>>> This patch works towards correcting this inconsistency by removing all uses
>>> of is_swap_pte() where we are already in a position where we perform
>>> pte_none()/pte_present() checks anyway or otherwise it is clearly logical
>>> to do so.
>>>
>>> We also take advantage of the fact that pte_swp_uffd_wp() is only set on
>>> swap entries.
>>>
>>> Additionally, update comments referencing to is_swap_pte() and
>>> non_swap_entry().
>>>
>>> No functional change intended.
>>>
>>> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>> ---
>>> fs/proc/task_mmu.c | 49 ++++++++++++++++++++++++-----------
>>> include/linux/userfaultfd_k.h | 3 +--
>>> mm/hugetlb.c | 6 ++---
>>> mm/internal.h | 6 ++---
>>> mm/khugepaged.c | 29 +++++++++++----------
>>> mm/migrate.c | 2 +-
>>> mm/mprotect.c | 43 ++++++++++++++----------------
>>> mm/mremap.c | 7 +++--
>>> mm/page_table_check.c | 13 ++++++----
>>> mm/page_vma_mapped.c | 31 +++++++++++-----------
>>> 10 files changed, 104 insertions(+), 85 deletions(-)
>>>
>>
>> <snip>
>>
>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>> index be20468fb5a9..a4e23818f37f 100644
>>> --- a/mm/page_vma_mapped.c
>>> +++ b/mm/page_vma_mapped.c
>>> @@ -16,6 +16,7 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
>>> static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>>> spinlock_t **ptlp)
>>> {
>>> + bool is_migration;
>>> pte_t ptent;
>>>
>>> if (pvmw->flags & PVMW_SYNC) {
>>> @@ -26,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>>> return !!pvmw->pte;
>>> }
>>>
>>> + is_migration = pvmw->flags & PVMW_MIGRATION;
>>> again:
>>> /*
>>> * It is important to return the ptl corresponding to pte,
>>> @@ -41,11 +43,14 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>>>
>>> ptent = ptep_get(pvmw->pte);
>>>
>>> - if (pvmw->flags & PVMW_MIGRATION) {
>>> - if (!is_swap_pte(ptent))
>>
>> Here, is_migration = true and either pte_none() or pte_present()
>> would return false, and ...
>>
>>> + if (pte_none(ptent)) {
>>> + return false;
>>> + } else if (pte_present(ptent)) {
>>> + if (is_migration)
>>> return false;
>>> - } else if (is_swap_pte(ptent)) {
>>> + } else if (!is_migration) {
>>> swp_entry_t entry;
>>> +
>>> /*
>>> * Handle un-addressable ZONE_DEVICE memory.
>>> *
>>> @@ -66,8 +71,6 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>>> if (!is_device_private_entry(entry) &&
>>> !is_device_exclusive_entry(entry))
>>> return false;
>>> - } else if (!pte_present(ptent)) {
>>> - return false;
>>
>> ... is_migration = false and !pte_present() is actually pte_none(),
>> because of the is_swap_pte() above the added !is_migration check.
>> So pte_none() should return false regardless of is_migration.
>
> I guess you were working this through :) well I decided to also just to
> double-check I got it right, maybe useful for you also :P -
>
> Previously:
>
> if (is_migration) {
> if (!is_swap_pte(ptent))
> return false;
> } else if (is_swap_pte(ptent)) {
> ... ZONE_DEVICE blah ...
> } else if (!pte_present(ptent)) {
> return false;
> }
>
> But is_swap_pte() is the same as !pte_none() && !pte_present(), so
> !is_swap_pte() is pte_none() || pte_present() by De Morgan's law:
>
> if (is_migration) {
> if (pte_none(ptent) || pte_present(ptent))
> return false;
> } else if (!pte_none(ptent) && !pte_present(ptent)) {
> ... ZONE_DEVICE blah ...
> } else if (!pte_present(ptent)) {
> return false;
> }
>
> In the last branch, we know (again by De Morgan's law) that either
> pte_none(ptent) or pte_present(ptent).. But we explicitly check for
> !pte_present(ptent) so this becomes:
>
> if (is_migration) {
> if (pte_none(ptent) || pte_present(ptent))
> return false;
> } else if (!pte_none(ptent) && !pte_present(ptent)) {
> ... ZONE_DEVICE blah ...
> } else if (pte_none(ptent)) {
> return false;
> }
>
> So we can generalise - regardless of is_migration, pte_none() returns false:
>
> if (pte_none(ptent)) {
> return false;
> } else if (is_migration) {
> if (pte_none(ptent) || pte_present(ptent))
> return false;
> } else if (!pte_none(ptent) && !pte_present(ptent)) {
> ... ZONE_DEVICE blah ...
> }
>
> Since we already check for pte_none() ahead of time, we can simplify again:
>
> if (pte_none(ptent)) {
> return false;
> } else if (is_migration) {
> if (pte_present(ptent))
> return false;
> } else if (!pte_present(ptent)) {
> ... ZONE_DEVICE blah ...
> }
>
> We can then put the pte_present() check in the outer branch:
>
> if (pte_none(ptent)) {
> return false;
> } else if (pte_present(ptent)) {
> if (is_migration)
> return false;
> } else if (!is_migration) {
> ... ZONE_DEVICE blah ...
> }
>
> Because previously an is_migration && !pte_present() case would result in no
> action here.
>
> Which is the code in this patch :)

Thanks again for spelling out the whole process.

>
>>
>> This is a nice cleanup. Thanks.
>>
>>> }
>>> spin_lock(*ptlp);
>>> if (unlikely(!pmd_same(*pmdvalp, pmdp_get_lockless(pvmw->pmd)))) {
>>> @@ -113,21 +116,17 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>>> return false;
>>>
>>> pfn = softleaf_to_pfn(entry);
>>> - } else if (is_swap_pte(ptent)) {
>>> - swp_entry_t entry;
>>> + } else if (pte_present(ptent)) {
>>> + pfn = pte_pfn(ptent);
>>> + } else {
>>> + const softleaf_t entry = softleaf_from_pte(ptent);
>>>
>>> /* Handle un-addressable ZONE_DEVICE memory */
>>> - entry = pte_to_swp_entry(ptent);
>>> - if (!is_device_private_entry(entry) &&
>>> - !is_device_exclusive_entry(entry))
>>> - return false;
>>> -
>>> - pfn = swp_offset_pfn(entry);
>>> - } else {
>>> - if (!pte_present(ptent))
>>
>> This !pte_present() is pte_none(). It seems that there should be
>
> Well this should be fine though as:
>
> const softleaf_t entry = softleaf_from_pte(ptent);
>
> /* Handle un-addressable ZONE_DEVICE memory */
> if (!softleaf_is_device_private(entry) &&
> !softleaf_is_device_exclusive(entry))
> return false;
>
> Still correctly handles none - as softleaf_from_pte() in case of pte_none() will
> be a none softleaf entry which will fail both of these tests.
>
> So excluding pte_none() as an explicit test here was part of the rework - we no
> longer have to do that.

Got it. Now my RB is yours. :)

>
>>
>> } else if (pte_none(ptent)) {
>> return false;
>> }
>>
>> before the above "} else {".
>>
>>> + if (!softleaf_is_device_private(entry) &&
>>> + !softleaf_is_device_exclusive(entry))
>>> return false;
>>>
>>> - pfn = pte_pfn(ptent);
>>> + pfn = softleaf_to_pfn(entry);
>>> }
>>>
>>> if ((pfn + pte_nr - 1) < pvmw->pfn)
>>> --
>>> 2.51.0
>>
>> Otherwise, LGTM. With the above issue addressed, feel free to
>> add Reviewed-by: Zi Yan <ziy@nvidia.com>
>
> Thanks!
>
>>
>> --
>> Best Regards,
>> Yan, Zi
>
> Cheers, Lorenzo

Best Regards,
Yan, Zi