linux-kernel - Re: [PATCH 2/3] userfaultfd: UFFDIO

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Date:   Thu, 14 Sep 2023 20:45:44 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org, aarcange@...hat.com,
        lokeshgidra@...gle.com, peterx@...hat.com, hughd@...gle.com,
        mhocko@...e.com, axelrasmussen@...gle.com, rppt@...nel.org,
        Liam.Howlett@...cle.com, jannh@...gle.com, zhangpeng362@...wei.com,
        bgeffon@...gle.com, kaleshsingh@...gle.com, ngeoffray@...gle.com,
        jdduke@...gle.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 2/3] userfaultfd: UFFDIO_REMAP uABI

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.

-- 
Cheers,

David / dhildenb