Re: [PATCH v2 5/9] mm/huge_memory: mark PMD mappings of the huge zero folio special

David Hildenbrand
2025-07-17T20:31:34Z

On 17.07.25 20:29, Lorenzo Stoakes wrote:
> On Thu, Jul 17, 2025 at 01:52:08PM +0200, David Hildenbrand wrote:
>> The huge zero folio is refcounted (+mapcounted -- is that a word?)
>> differently than "normal" folios, similarly (but different) to the ordinary
>> shared zeropage.
>
> Yeah, I sort of wonder if we shouldn't just _not_ do any of that with zero
> pages?

I wish we could get rid of the weird refcounting of the huge zero folio
and get rid of the shrinker. But as long as the shrinker exists, I'm
afraid that weird per-process refcounting must stay.

>
> But for some reason the huge zero page wants to exist or not exist based on
> usage for one. Which is stupid to me.

Yes, I will try at some point (once we have the static huge zero folio)
to remove the shrinker part and make it always static. Well, at least
for reasonable architectures.

>
>>
>> For this reason, we special-case these pages in
>> vm_normal_page*/vm_normal_folio*, and only allow selected callers to
>> still use them (e.g., GUP can still take a reference on them).
>>
>> vm_normal_page_pmd() already filters out the huge zero folio. However,
>> so far we are not marking it as special like we do with the ordinary
>> shared zeropage. Let's mark it as special, so we can further refactor
>> vm_normal_page_pmd() and vm_normal_page().
>>
>> While at it, update the doc regarding the shared zero folios.
>
> Hmm I wonder how this will interact with the static PMD series at [0]?

No, it shouldn't.

>
> I wonder if more use of that might result in some weirdness with refcounting
> etc.?

I don't think so.

>
> Also, that series was (though I reviewed against it) moving stuff that
> references the huge zero folio out of there, but also generally allows
> access and mapping of this folio via largest_zero_folio() so not only via
> insert_pmd().
>
> So we're going to end up with mappings of this that are not marked special
> that are potentially going to have refcount/mapcount manipulation that
> contradict what you're doing here perhaps?

I don't think so. It's just like having the existing static (small)
shared zeropage where the same rules about refcounting+mapcounting apply.

>
> [0]: https://lore.kernel.org/all/20250707142319.319642-1-kernel@pankajraghav.com/
>
>>
>> Reviewed-by: Oscar Salvador <osalvador@suse.de>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>
> I looked thorugh places that use vm_normal_page_pm() (other than decl of
> function):
>
> fs/proc/task_mmu.c - seems to handle NULL page correctly + still undertsands zero page
> mm/pagewalk.c - correctly handles NULL page + huge zero page
> mm/huge_memory.c - can_change_pmd_writable() correctly returns false.
>
> And all seems to work wtih this change.
>
> Overall, other than concerns above + nits below LGTM, we should treat all
> the zero folios the same in this regard, so:
>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks!

>
>> ---
>> mm/huge_memory.c | 5 ++++-
>> mm/memory.c | 14 +++++++++-----
>> 2 files changed, 13 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index db08c37b87077..3f9a27812a590 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1320,6 +1320,7 @@ static void set_huge_zero_folio(pgtable_t pgtable, struct mm_struct *mm,
>> {
>> pmd_t entry;
>> entry = folio_mk_pmd(zero_folio, vma->vm_page_prot);
>> + entry = pmd_mkspecial(entry);
>> pgtable_trans_huge_deposit(mm, pmd, pgtable);
>> set_pmd_at(mm, haddr, pmd, entry);
>> mm_inc_nr_ptes(mm);
>> @@ -1429,7 +1430,9 @@ static vm_fault_t insert_pmd(struct vm_area_struct *vma, unsigned long addr,
>> if (fop.is_folio) {
>> entry = folio_mk_pmd(fop.folio, vma->vm_page_prot);
>>
>> - if (!is_huge_zero_folio(fop.folio)) {
>> + if (is_huge_zero_folio(fop.folio)) {
>> + entry = pmd_mkspecial(entry);
>> + } else {
>> folio_get(fop.folio);
>> folio_add_file_rmap_pmd(fop.folio, &fop.folio->page, vma);
>> add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR);
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 92fd18a5d8d1f..173eb6267e0ac 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -537,7 +537,13 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
>> *
>> * "Special" mappings do not wish to be associated with a "struct page" (either
>> * it doesn't exist, or it exists but they don't want to touch it). In this
>> - * case, NULL is returned here. "Normal" mappings do have a struct page.
>> + * case, NULL is returned here. "Normal" mappings do have a struct page and
>> + * are ordinarily refcounted.
>> + *
>> + * Page mappings of the shared zero folios are always considered "special", as
>> + * they are not ordinarily refcounted. However, selected page table walkers
>> + * (such as GUP) can still identify these mappings and work with the
>> + * underlying "struct page".
>
> I feel like we need more detail or something more explicit about what 'not
> ordinary' refcounting constitutes. This is a bit vague.

Hm, I am not sure this is the correct place to document that. But let me
see if I can come up with something reasonable

(like, the refcount and mapcount of these folios is never adjusted when
mapping them into page tables)

>
>> *
>> * There are 2 broad cases. Firstly, an architecture may define a pte_special()
>> * pte bit, in which case this function is trivial. Secondly, an architecture
>> @@ -567,9 +573,8 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
>> *
>> * VM_MIXEDMAP mappings can likewise contain memory with or without "struct
>> * page" backing, however the difference is that _all_ pages with a struct
>> - * page (that is, those where pfn_valid is true) are refcounted and considered
>> - * normal pages by the VM. The only exception are zeropages, which are
>> - * *never* refcounted.
>> + * page (that is, those where pfn_valid is true, except the shared zero
>> + * folios) are refcounted and considered normal pages by the VM.
>> *
>> * The disadvantage is that pages are refcounted (which can be slower and
>> * simply not an option for some PFNMAP users). The advantage is that we
>> @@ -649,7 +654,6 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
>
> You know I"m semi-ashamed to admit I didn't even know this function
> exists. But yikes that we have a separate function like this just for PMDs.

It's a bit new-ish :)

--
Cheers,

David / dhildenb