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From:   Alistair Popple <apopple@nvidia.com>
To:     Peter Xu <peterx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Jerome Glisse <jglisse@redhat.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Axel Rasmussen <axelrasmussen@google.com>
Subject: Re: [PATCH v8 02/23] mm: Teach core mm about pte markers
Date:   Tue, 12 Apr 2022 11:22:01 +1000
References: <20220405014646.13522-1-peterx@redhat.com>
 <20220405014833.14015-1-peterx@redhat.com>
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--=-=-=
Content-Type: text/plain
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I've been reviewing existing pte_none() call sites and noticed the following:

From khugepaged_scan_pmd():

pte_t pteval = *_pte;
if (is_swap_pte(pteval)) {
    if (++unmapped <= khugepaged_max_ptes_swap) {
        /*
* Always be strict with uffd-wp
* enabled swap entries.  Please see
* comment below for pte_uffd_wp().
         */
        if (pte_swp_uffd_wp(pteval)) {
            result = SCAN_PTE_UFFD_WP;
            goto out_unmap;
        }
        continue;
    } else {
        result = SCAN_EXCEED_SWAP_PTE;
        count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
        goto out_unmap;
    }
}
if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
    if (!userfaultfd_armed(vma) &&
        ++none_or_zero <= khugepaged_max_ptes_none) {
        continue;
    } else {
        result = SCAN_EXCEED_NONE_PTE;
        count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
        goto out_unmap;
    }
}

I think the above could encounter a marker PTE right? So the behviour would be
wrong in that case. As I understand things the is_swap_pte() path will be taken
rather than pte_none() but in the absence of any special handling shouldn't
marker PTE's mostly be treated as pte_none() here?

I think you need to s/pte_none/pte_none_mostly/ here and swap the order of
conditionals around.

- Alistair

Peter Xu <peterx@redhat.com> writes:

> This patch still does not use pte marker in any way, however it teaches the
> core mm about the pte marker idea.
>
> For example, handle_pte_marker() is introduced that will parse and handle all
> the pte marker faults.
>
> Many of the places are more about commenting it up - so that we know there's
> the possibility of pte marker showing up, and why we don't need special code
> for the cases.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  fs/userfaultfd.c | 10 ++++++----
>  mm/filemap.c     |  5 +++++
>  mm/hmm.c         |  2 +-
>  mm/memcontrol.c  |  8 ++++++--
>  mm/memory.c      | 23 +++++++++++++++++++++++
>  mm/mincore.c     |  3 ++-
>  mm/mprotect.c    |  3 +++
>  7 files changed, 46 insertions(+), 8 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index aa0c47cb0d16..8b4a94f5a238 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -249,9 +249,10 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
>
>  	/*
>  	 * Lockless access: we're in a wait_event so it's ok if it
> -	 * changes under us.
> +	 * changes under us.  PTE markers should be handled the same as none
> +	 * ptes here.
>  	 */
> -	if (huge_pte_none(pte))
> +	if (huge_pte_none_mostly(pte))
>  		ret = true;
>  	if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
>  		ret = true;
> @@ -330,9 +331,10 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
>  	pte = pte_offset_map(pmd, address);
>  	/*
>  	 * Lockless access: we're in a wait_event so it's ok if it
> -	 * changes under us.
> +	 * changes under us.  PTE markers should be handled the same as none
> +	 * ptes here.
>  	 */
> -	if (pte_none(*pte))
> +	if (pte_none_mostly(*pte))
>  		ret = true;
>  	if (!pte_write(*pte) && (reason & VM_UFFD_WP))
>  		ret = true;
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 3a5ffb5587cd..ef77dae8c28d 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3382,6 +3382,11 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
>  		vmf->pte += xas.xa_index - last_pgoff;
>  		last_pgoff = xas.xa_index;
>
> +		/*
> +		 * NOTE: If there're PTE markers, we'll leave them to be
> +		 * handled in the specific fault path, and it'll prohibit the
> +		 * fault-around logic.
> +		 */
>  		if (!pte_none(*vmf->pte))
>  			goto unlock;
>
> diff --git a/mm/hmm.c b/mm/hmm.c
> index af71aac3140e..3fd3242c5e50 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -239,7 +239,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>  	pte_t pte = *ptep;
>  	uint64_t pfn_req_flags = *hmm_pfn;
>
> -	if (pte_none(pte)) {
> +	if (pte_none_mostly(pte)) {
>  		required_fault =
>  			hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0);
>  		if (required_fault)
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7a08737bac4b..08af97c73f0f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5644,10 +5644,14 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
>
>  	if (pte_present(ptent))
>  		page = mc_handle_present_pte(vma, addr, ptent);
> +	else if (pte_none_mostly(ptent))
> +		/*
> +		 * PTE markers should be treated as a none pte here, separated
> +		 * from other swap handling below.
> +		 */
> +		page = mc_handle_file_pte(vma, addr, ptent);
>  	else if (is_swap_pte(ptent))
>  		page = mc_handle_swap_pte(vma, ptent, &ent);
> -	else if (pte_none(ptent))
> -		page = mc_handle_file_pte(vma, addr, ptent);
>
>  	if (!page && !ent.val)
>  		return ret;
> diff --git a/mm/memory.c b/mm/memory.c
> index 2c5d1bb4694f..3f396241a7db 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -100,6 +100,8 @@ struct page *mem_map;
>  EXPORT_SYMBOL(mem_map);
>  #endif
>
> +static vm_fault_t do_fault(struct vm_fault *vmf);
> +
>  /*
>   * A number of key systems in x86 including ioremap() rely on the assumption
>   * that high_memory defines the upper bound on direct map memory, then end
> @@ -1415,6 +1417,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>  			if (!should_zap_page(details, page))
>  				continue;
>  			rss[mm_counter(page)]--;
> +		} else if (is_pte_marker_entry(entry)) {
> +			/* By default, simply drop all pte markers when zap */
>  		} else if (is_hwpoison_entry(entry)) {
>  			if (!should_zap_cows(details))
>  				continue;
> @@ -3555,6 +3559,23 @@ static inline bool should_try_to_free_swap(struct page *page,
>  		page_count(page) == 2;
>  }
>
> +static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> +{
> +	swp_entry_t entry = pte_to_swp_entry(vmf->orig_pte);
> +	unsigned long marker = pte_marker_get(entry);
> +
> +	/*
> +	 * PTE markers should always be with file-backed memories, and the
> +	 * marker should never be empty.  If anything weird happened, the best
> +	 * thing to do is to kill the process along with its mm.
> +	 */
> +	if (WARN_ON_ONCE(vma_is_anonymous(vmf->vma) || !marker))
> +		return VM_FAULT_SIGBUS;
> +
> +	/* TODO: handle pte markers */
> +	return 0;
> +}
> +
>  /*
>   * We enter with non-exclusive mmap_lock (to exclude vma changes,
>   * but allow concurrent faults), and pte mapped but not yet locked.
> @@ -3592,6 +3613,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  			ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
>  		} else if (is_hwpoison_entry(entry)) {
>  			ret = VM_FAULT_HWPOISON;
> +		} else if (is_pte_marker_entry(entry)) {
> +			ret = handle_pte_marker(vmf);
>  		} else {
>  			print_bad_pte(vma, vmf->address, vmf->orig_pte, NULL);
>  			ret = VM_FAULT_SIGBUS;
> diff --git a/mm/mincore.c b/mm/mincore.c
> index f4f627325e12..fa200c14185f 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -122,7 +122,8 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  	for (; addr != end; ptep++, addr += PAGE_SIZE) {
>  		pte_t pte = *ptep;
>
> -		if (pte_none(pte))
> +		/* We need to do cache lookup too for pte markers */
> +		if (pte_none_mostly(pte))
>  			__mincore_unmapped_range(addr, addr + PAGE_SIZE,
>  						 vma, vec);
>  		else if (pte_present(pte))
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 56060acdabd3..709a6f73b764 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -188,6 +188,9 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  					newpte = pte_swp_mksoft_dirty(newpte);
>  				if (pte_swp_uffd_wp(oldpte))
>  					newpte = pte_swp_mkuffd_wp(newpte);
> +			} else if (is_pte_marker_entry(entry)) {
> +				/* Skip it, the same as none pte */
> +				continue;
>  			} else {
>  				newpte = oldpte;
>  			}

--=-=-=--