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Date:   Thu, 21 Sep 2023 20:17:45 +0200
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Subject: Re: [PATCH 2/3] userfaultfd: UFFDIO_REMAP uABI
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Matthew Wilcox <willy@infradead.org>, akpm@linux-foundation.org,
        viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org,
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        rppt@kernel.org, Liam.Howlett@oracle.com, jannh@google.com,
        zhangpeng362@huawei.com, bgeffon@google.com,
        kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        kernel-team@android.com
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <CAJuCfpGqt1V5puRMhLkjG6F2T4xtsDY8qy--ZfBPNL9kxPyWtg@mail.gmail.com>
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On 21.09.23 20:04, Suren Baghdasaryan wrote:
> On Thu, Sep 14, 2023 at 6:45 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 14.09.23 20:43, David Hildenbrand wrote:
>>> On 14.09.23 20:11, Matthew Wilcox wrote:
>>>> On Thu, Sep 14, 2023 at 08:26:12AM -0700, Suren Baghdasaryan wrote:
>>>>> +++ b/include/linux/userfaultfd_k.h
>>>>> @@ -93,6 +93,23 @@ extern int mwriteprotect_range(struct mm_struct *dst_mm,
>>>>>     extern long uffd_wp_range(struct vm_area_struct *vma,
>>>>>                        unsigned long start, unsigned long len, bool enable_wp);
>>>>>
>>>>> +/* remap_pages */
>>>>> +extern void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2);
>>>>> +extern void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2);
>>>>> +extern ssize_t remap_pages(struct mm_struct *dst_mm,
>>>>> +                      struct mm_struct *src_mm,
>>>>> +                      unsigned long dst_start,
>>>>> +                      unsigned long src_start,
>>>>> +                      unsigned long len, __u64 flags);
>>>>> +extern int remap_pages_huge_pmd(struct mm_struct *dst_mm,
>>>>> +                           struct mm_struct *src_mm,
>>>>> +                           pmd_t *dst_pmd, pmd_t *src_pmd,
>>>>> +                           pmd_t dst_pmdval,
>>>>> +                           struct vm_area_struct *dst_vma,
>>>>> +                           struct vm_area_struct *src_vma,
>>>>> +                           unsigned long dst_addr,
>>>>> +                           unsigned long src_addr);
>>>>
>>>> Drop the 'extern' markers from function declarations.
>>>>
>>>>> +int remap_pages_huge_pmd(struct mm_struct *dst_mm,
>>>>> +                    struct mm_struct *src_mm,
>>>>> +                    pmd_t *dst_pmd, pmd_t *src_pmd,
>>>>> +                    pmd_t dst_pmdval,
>>>>> +                    struct vm_area_struct *dst_vma,
>>>>> +                    struct vm_area_struct *src_vma,
>>>>> +                    unsigned long dst_addr,
>>>>> +                    unsigned long src_addr)
>>>>> +{
>>>>> +   pmd_t _dst_pmd, src_pmdval;
>>>>> +   struct page *src_page;
>>>>> +   struct anon_vma *src_anon_vma, *dst_anon_vma;
>>>>> +   spinlock_t *src_ptl, *dst_ptl;
>>>>> +   pgtable_t pgtable;
>>>>> +   struct mmu_notifier_range range;
>>>>> +
>>>>> +   src_pmdval = *src_pmd;
>>>>> +   src_ptl = pmd_lockptr(src_mm, src_pmd);
>>>>> +
>>>>> +   BUG_ON(!pmd_trans_huge(src_pmdval));
>>>>> +   BUG_ON(!pmd_none(dst_pmdval));
>>>>> +   BUG_ON(!spin_is_locked(src_ptl));
>>>>> +   mmap_assert_locked(src_mm);
>>>>> +   mmap_assert_locked(dst_mm);
>>>>> +   BUG_ON(src_addr & ~HPAGE_PMD_MASK);
>>>>> +   BUG_ON(dst_addr & ~HPAGE_PMD_MASK);
>>>>> +
>>>>> +   src_page = pmd_page(src_pmdval);
>>>>> +   BUG_ON(!PageHead(src_page));
>>>>> +   BUG_ON(!PageAnon(src_page));
>>>>
>>>> Better to add a src_folio = page_folio(src_page);
>>>> and then folio_test_anon() here.
>>>>
>>>>> +   if (unlikely(page_mapcount(src_page) != 1)) {
>>>>
>>>> Brr, this is going to miss PTE mappings of this folio.  I think you
>>>> actually want folio_mapcount() instead, although it'd be more efficient
>>>> to look at folio->_entire_mapcount == 1 and _nr_pages_mapped == 0.
>>>> Not wure what a good name for that predicate would be.
>>>
>>> We have
>>>
>>>     * It only works on non shared anonymous pages because those can
>>>     * be relocated without generating non linear anon_vmas in the rmap
>>>     * code.
>>>     *
>>>     * It provides a zero copy mechanism to handle userspace page faults.
>>>     * The source vma pages should have mapcount == 1, which can be
>>>     * enforced by using madvise(MADV_DONTFORK) on src vma.
>>>
>>> Use PageAnonExclusive(). As long as KSM is not involved and you don't
>>> use fork(), that flag should be good enough for that use case here.
>>>
>> ... and similarly don't do any of that swapcount stuff and only check if
>> the swap pte is anon exclusive.
> 
> I'm preparing v2 and this is the only part left for me to address but
> I'm not clear how. David, could you please clarify how I should be
> checking swap pte to be exclusive without swapcount?

If you have a real swp pte (not a non-swap pte like migration entries) 
you should be able to just use pte_swp_exclusive().

-- 
Cheers,

David / dhildenb