On 06/09/2024 10:00, Dev Jain wrote:
>
> On 9/6/24 14:13, Ryan Roberts wrote:
>> On 06/09/2024 08:05, Dev Jain wrote:
>>> On 9/5/24 18:44, Ryan Roberts wrote:
>>>> On 04/09/2024 11:09, Dev Jain wrote:
>>>>> Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and
>>>>> replace it with a PMD-mapped THP. Change the helpers introduced in the
>>>>> previous patch to flush TLB entry corresponding to the hugezeropage,
>>>>> and preserve PMD uffd-wp marker. In case of failure, fallback to
>>>>> splitting the PMD.
>>>>>
>>>>> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>>>>> ---
>>>>> include/linux/huge_mm.h | 6 ++++
>>>>> mm/huge_memory.c | 79 +++++++++++++++++++++++++++++++++++------
>>>>> mm/memory.c | 5 +--
>>>>> 3 files changed, 78 insertions(+), 12 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>>> index e25d9ebfdf89..fdd2cf473a3c 100644
>>>>> --- a/include/linux/huge_mm.h
>>>>> +++ b/include/linux/huge_mm.h
>>>>> @@ -9,6 +9,12 @@
>>>>> #include <linux/kobject.h>
>>>>> vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf);
>>>>> +vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
>>>>> + unsigned long haddr, struct folio **foliop,
>>>>> + unsigned long addr);
>>>>> +void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>>>>> + struct vm_area_struct *vma, unsigned long haddr,
>>>>> + pgtable_t pgtable);
>>>> I don't think you are using either of these outside of huge_memory.c, so not
>>>> sure you need to declare them here or make them non-static?
>>> As pointed out by Kirill, you are right, I forgot to drop these from my previous
>>> approach.
>>>
>>>>> int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>>>>> pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
>>>>> struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma);
>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>> index 58125fbcc532..150163ad77d3 100644
>>>>> --- a/mm/huge_memory.c
>>>>> +++ b/mm/huge_memory.c
>>>>> @@ -943,9 +943,9 @@ unsigned long thp_get_unmapped_area(struct file *filp,
>>>>> unsigned long addr,
>>>>> }
>>>>> EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
>>>>> -static vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct
>>>>> vm_area_struct *vma,
>>>>> - unsigned long haddr, struct folio **foliop,
>>>>> - unsigned long addr)
>>>>> +vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
>>>>> + unsigned long haddr, struct folio **foliop,
>>>>> + unsigned long addr)
>>>>> {
>>>>> struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
>>>>> @@ -984,21 +984,29 @@ static void __thp_fault_success_stats(struct
>>>>> vm_area_struct *vma, int order)
>>>>> count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
>>>>> }
>>>>> -static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>>>>> - struct vm_area_struct *vma, unsigned long haddr,
>>>>> - pgtable_t pgtable)
>>>>> +void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>>>>> + struct vm_area_struct *vma, unsigned long haddr,
>>>>> + pgtable_t pgtable)
>>>>> {
>>>>> - pmd_t entry;
>>>>> + pmd_t entry, old_pmd;
>>>>> + bool is_pmd_none = pmd_none(*vmf->pmd);
>>>>> entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
>>>>> entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>>>>> folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
>>>>> folio_add_lru_vma(folio, vma);
>>>>> - pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
>>>>> + if (!is_pmd_none) {
>>>>> + old_pmd = pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
>>>>> + if (pmd_uffd_wp(old_pmd))
>>>>> + entry = pmd_mkuffd_wp(entry);
>>>> I don't really get this; entry is writable, so I wouldn't expect to also be
>>>> setting uffd-wp here? That combination is not allowed and is checked for in
>>>> page_table_check_pte_flags().
>>>>
>>>> It looks like you expect to get here in the uffd-wp-async case, which used to
>>>> cause the pmd to be split to ptes. I'm guessing (but don't know for sure) that
>>>> would cause the uffd-wp bit to be set in each of the new ptes, then during
>>>> fallback to handling the wp fault on the pte, uffd would handle it?
>>> I guess you are correct; I missed the WARN_ON() in page_table_check_pmd_flags(),
>>> but I did see, if I read the uffd code correctly, that mfill_atomic() will just
>>> return in case of pmd_trans_huge(*dst_pmd) while doing a uffd_copy to the
>>> destination
>>> location. So preserving pmd_uffd_wp is useless in case a THP is mapped, but I
>>> did not
>>> know that in fact it is supposed to be an invalid combination. So, I will
>>> drop it,
>>> unless someone has some other objection.
>> So what's the correct way to handle uffd-wp-async in wp_huge_pmd()? Just split
>> it? If so, you can revert your changes in memory.c.
>
> I think so.
>
>>
>>>>> + }
>>>>> + if (pgtable)
>>>>> + pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
>>>> Should this call be moved outside of here? It doesn't really feel like it
>>>> belongs. Could it be called before calling map_pmd_thp() for the site that
>>>> has a
>>>> pgtable?
>>> Every other place I checked, they are doing this: make the entry -> deposit
>>> pgtable ->
>>> set_pmd_at(). I guess the general flow is to do the deposit based on the old
>>> pmd, before
>>> setting the new one. Which implies: I should at least move this check before
>>> I call
>>> pmdp_huge_clear_flush(). And, since vmf->pmd and creating the new entry has no
>>> relation,
>>> I am inclined to do what you are saying.
>> pgtable_trans_huge_deposit() is just adding the pgtable to a list so that if the
>> THP needs to be split in future, then we have preallocated the pte pgtable so
>> the operation can't fail.
>
> Yes.
>
>> And enqueing it is just under the protection of the
>> PTL as far as I can tell. So I think the only ordering requirement is that you
>> both set the pmd and deposit the pgtable under the lock (without dropping it in
>> between). So you can deposit either before or after map_pmd_thp().
>
> Yes I'll do that before.
>
>> And
>> pmdp_huge_clear_flush() is irrelavent, I think?
>
> You mean, in this context? Everywhere, pgtable deposit uses the old pmd
> value to be replaced as its input, that is, it is called before set_pmd_at().
> So calling pgtable deposit after clear_flush() will violate this ordering.
> I do not think this ordering is really required but I'd rather be safe :)
The pmd pointer is just used to get the pmd table (the pointer points to an
entry inside the table so its just a case of backwards aligning the pointer).
The pointer is never dereferenced, so the value of the entry is irrelevant.
>
>>
>>>>> set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
>>>>> update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
>>>>> add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>>>>> - mm_inc_nr_ptes(vma->vm_mm);
>>>>> + if (is_pmd_none)
>>>>> + mm_inc_nr_ptes(vma->vm_mm);
>>>>> }
>>>>> static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>>>>> @@ -1576,6 +1584,50 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
>>>>> spin_unlock(vmf->ptl);
>>>>> }
>>>>> +static vm_fault_t do_huge_zero_wp_pmd_locked(struct vm_fault *vmf,
>>>>> + unsigned long haddr,
>>>>> + struct folio *folio)
>>>>> +{
>>>>> + struct vm_area_struct *vma = vmf->vma;
>>>>> + vm_fault_t ret = 0;
>>>>> +
>>>>> + ret = check_stable_address_space(vma->vm_mm);
>>>>> + if (ret)
>>>>> + goto out;
>>>>> + map_pmd_thp(folio, vmf, vma, haddr, NULL);
>>>>> +out:
>>>>> + return ret;
>>>>> +}
>>>>> +
>>>>> +static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf, unsigned long
>>>>> haddr)
>>>>> +{
>>>>> + struct vm_area_struct *vma = vmf->vma;
>>>>> + gfp_t gfp = vma_thp_gfp_mask(vma);
>>>>> + struct mmu_notifier_range range;
>>>>> + struct folio *folio = NULL;
>>>>> + vm_fault_t ret = 0;
>>>>> +
>>>>> + ret = thp_fault_alloc(gfp, HPAGE_PMD_ORDER, vma, haddr, &folio,
>>>>> + vmf->address);
>>>> Just checking: the PTE table was already allocated during the read fault,
>>>> right?
>>>> So we don't have to allocate it here.
>>> Correct, that happens in set_huge_zero_folio(). Thanks for checking.
>>>
>>>>> + if (ret)
>>>>> + goto out;
>>>>> +
>>>>> + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, haddr,
>>>>> + haddr + HPAGE_PMD_SIZE);
>>>>> + mmu_notifier_invalidate_range_start(&range);
>>>>> + vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>>>>> + if (unlikely(!pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
>>>>> + goto unlock;
>>>>> + ret = do_huge_zero_wp_pmd_locked(vmf, haddr, folio);
>>>>> + if (!ret)
>>>>> + __thp_fault_success_stats(vma, HPAGE_PMD_ORDER);
>>>>> +unlock:
>>>>> + spin_unlock(vmf->ptl);
>>>>> + mmu_notifier_invalidate_range_end(&range);
>>>> I'll confess I don't understand all the mmu notifier rules.
>>> I confess the same :)
>>>
>>>> But the doc at
>>>> Documentation/mm/mmu_notifier.rst implies that the notification must be done
>>>> while holding the PTL. Although that's not how wp_page_copy(). Are you
>>>> confident
>>>> what you have done is correct?
>>> Everywhere else, invalidate_range_end() is getting called after dropping the
>>> lock,
>>> one reason is that it has a might_sleep(), and therefore we cannot call it while
>>> holding a spinlock. I still don't know what exactly these calls mean...but I
>>> think
>>> what I have done is correct.
>> High level; they are notifying secondary MMUs (e.g. IOMMU) of a change so the
>> tables of those secondary MMUs can be kept in sync. I don't understand all the
>> ordering requirement details though.
>>
>> I think what you have is probably correct, would be good for someone that knows
>> what they are talking about to confirm though :)
>
> Exactly.
>
>>
>>>> Thanks,
>>>> Ryan
>>>>
>>>>> +out:
>>>>> + return ret;
>>>>> +}
>>>>> +
>>>>> vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>>>>> {
>>>>> const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
>>>>> @@ -1588,8 +1640,15 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>>>>> vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
>>>>> VM_BUG_ON_VMA(!vma->anon_vma, vma);
>>>>> - if (is_huge_zero_pmd(orig_pmd))
>>>>> + if (is_huge_zero_pmd(orig_pmd)) {
>>>>> + vm_fault_t ret = do_huge_zero_wp_pmd(vmf, haddr);
>>>>> +
>>>>> + if (!(ret & VM_FAULT_FALLBACK))
>>>>> + return ret;
>>>>> +
>>>>> + /* Fallback to splitting PMD if THP cannot be allocated */
>>>>> goto fallback;
>>>>> + }
>>>>> spin_lock(vmf->ptl);
>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>> index 3c01d68065be..c081a25f5173 100644
>>>>> --- a/mm/memory.c
>>>>> +++ b/mm/memory.c
>>>>> @@ -5409,9 +5409,10 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault
>>>>> *vmf)
>>>>> if (vma_is_anonymous(vma)) {
>>>>> if (likely(!unshare) &&
>>>>> userfaultfd_huge_pmd_wp(vma, vmf->orig_pmd)) {
>>>>> - if (userfaultfd_wp_async(vmf->vma))
>>>>> + if (!userfaultfd_wp_async(vmf->vma))
>>>>> + return handle_userfault(vmf, VM_UFFD_WP);
>>>>> + if (!is_huge_zero_pmd(vmf->orig_pmd))
>>>>> goto split;
>>>>> - return handle_userfault(vmf, VM_UFFD_WP);
>>>>> }
>>>>> return do_huge_pmd_wp_page(vmf);
>>>>> }
