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Date:   Wed, 7 Dec 2022 16:12:31 -0800
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Subject: Re: [PATCH v2 09/10] mm/hugetlb: Introduce hugetlb_walk()
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To:     Peter Xu <peterx@redhat.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
CC:     Jann Horn <jannh@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        James Houghton <jthoughton@google.com>,
        Rik van Riel <riel@surriel.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        "Mike Kravetz" <mike.kravetz@oracle.com>,
        David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Muchun Song <songmuchun@bytedance.com>
References: <20221207203034.650899-1-peterx@redhat.com>
 <20221207203156.651077-1-peterx@redhat.com>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <20221207203156.651077-1-peterx@redhat.com>
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On 12/7/22 12:31, Peter Xu wrote:
> huge_pte_offset() is the main walker function for hugetlb pgtables.  The
> name is not really representing what it does, though.
> 
> Instead of renaming it, introduce a wrapper function called hugetlb_walk()
> which will use huge_pte_offset() inside.  Assert on the locks when walking
> the pgtable.
> 
> Note, the vma lock assertion will be a no-op for private mappings.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   fs/hugetlbfs/inode.c    |  4 +---
>   fs/userfaultfd.c        |  6 ++----
>   include/linux/hugetlb.h | 39 +++++++++++++++++++++++++++++++++++++++
>   mm/hugetlb.c            | 32 +++++++++++++-------------------
>   mm/page_vma_mapped.c    |  2 +-
>   mm/pagewalk.c           |  4 +---
>   6 files changed, 57 insertions(+), 30 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index fdb16246f46e..48f1a8ad2243 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -388,9 +388,7 @@ static bool hugetlb_vma_maps_page(struct vm_area_struct *vma,
>   {
>   	pte_t *ptep, pte;
>   
> -	ptep = huge_pte_offset(vma->vm_mm, addr,
> -			huge_page_size(hstate_vma(vma)));
> -
> +	ptep = hugetlb_walk(vma, addr, huge_page_size(hstate_vma(vma)));
>   	if (!ptep)
>   		return false;
>   
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index a602f008dde5..f31fe1a9f4c5 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -237,14 +237,12 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
>   					 unsigned long flags,
>   					 unsigned long reason)
>   {
> -	struct mm_struct *mm = ctx->mm;
>   	pte_t *ptep, pte;
>   	bool ret = true;
>   
> -	mmap_assert_locked(mm);
> -
> -	ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
> +	mmap_assert_locked(ctx->mm);
>   
> +	ptep = hugetlb_walk(vma, address, vma_mmu_pagesize(vma));
>   	if (!ptep)
>   		goto out;
>   
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 81efd9b9baa2..1c20cbbf3d22 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -2,6 +2,7 @@
>   #ifndef _LINUX_HUGETLB_H
>   #define _LINUX_HUGETLB_H
>   
> +#include <linux/mm.h>
>   #include <linux/mm_types.h>
>   #include <linux/mmdebug.h>
>   #include <linux/fs.h>
> @@ -196,6 +197,11 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
>    * huge_pte_offset(): Walk the hugetlb pgtable until the last level PTE.
>    * Returns the pte_t* if found, or NULL if the address is not mapped.
>    *
> + * IMPORTANT: we should normally not directly call this function, instead
> + * this is only a common interface to implement arch-specific walker.
> + * Please consider using the hugetlb_walk() helper to make sure of the
> + * correct locking is satisfied.

Or:

"Please use hugetlb_walk() instead, because that will attempt to verify
the locking for you."

> + *
>    * Since this function will walk all the pgtable pages (including not only
>    * high-level pgtable page, but also PUD entry that can be unshared
>    * concurrently for VM_SHARED), the caller of this function should be
> @@ -1229,4 +1235,37 @@ bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr);
>   #define flush_hugetlb_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
>   #endif
>   
> +static inline bool
> +__vma_shareable_flags_pmd(struct vm_area_struct *vma)
> +{
> +	return vma->vm_flags & (VM_MAYSHARE | VM_SHARED) &&
> +		vma->vm_private_data;
> +}
> +
> +/*
> + * Safe version of huge_pte_offset() to check the locks.  See comments
> + * above huge_pte_offset().
> + */

It is odd to say that functionA() is a safe version of functionB(), if the
names are completely different.

At this point, it is very clear that huge_pte_offset() should be renamed.
I'd suggest something like one of these:

     __hugetlb_walk()
     hugetlb_walk_raw()

> +static inline pte_t *
> +hugetlb_walk(struct vm_area_struct *vma, unsigned long addr, unsigned long sz)
> +{
> +#if defined(CONFIG_HUGETLB_PAGE) && \
> +	defined(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) && defined(CONFIG_LOCKDEP)
> +	struct hugetlb_vma_lock *vma_lock = vma->vm_private_data;
> +
> +	/*
> +	 * If pmd sharing possible, locking needed to safely walk the
> +	 * hugetlb pgtables.  More information can be found at the comment
> +	 * above huge_pte_offset() in the same file.
> +	 *
> +	 * NOTE: lockdep_is_held() is only defined with CONFIG_LOCKDEP.
> +	 */
> +	if (__vma_shareable_flags_pmd(vma))
> +		WARN_ON_ONCE(!lockdep_is_held(&vma_lock->rw_sema) &&
> +			     !lockdep_is_held(
> +				 &vma->vm_file->f_mapping->i_mmap_rwsem));
> +#endif
> +	return huge_pte_offset(vma->vm_mm, addr, sz);
> +}

Let's please not slice up C functions with ifdefs. Instead, stick to the
standard approach of

#ifdef X
functionC()
{
	...implementation
}
#else
functionC()
{
	...simpler or shorter or stub implementation
}

> +
>   #endif /* _LINUX_HUGETLB_H */
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f42399522805..e3500c087893 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4814,7 +4814,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>   	} else {
>   		/*
>   		 * For shared mappings the vma lock must be held before
> -		 * calling huge_pte_offset in the src vma. Otherwise, the
> +		 * calling hugetlb_walk() in the src vma. Otherwise, the
>   		 * returned ptep could go away if part of a shared pmd and
>   		 * another thread calls huge_pmd_unshare.
>   		 */
> @@ -4824,7 +4824,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>   	last_addr_mask = hugetlb_mask_last_page(h);
>   	for (addr = src_vma->vm_start; addr < src_vma->vm_end; addr += sz) {
>   		spinlock_t *src_ptl, *dst_ptl;
> -		src_pte = huge_pte_offset(src, addr, sz);
> +		src_pte = hugetlb_walk(src_vma, addr, sz);
>   		if (!src_pte) {
>   			addr |= last_addr_mask;
>   			continue;
> @@ -5028,7 +5028,7 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
>   	hugetlb_vma_lock_write(vma);
>   	i_mmap_lock_write(mapping);
>   	for (; old_addr < old_end; old_addr += sz, new_addr += sz) {
> -		src_pte = huge_pte_offset(mm, old_addr, sz);
> +		src_pte = hugetlb_walk(vma, old_addr, sz);
>   		if (!src_pte) {
>   			old_addr |= last_addr_mask;
>   			new_addr |= last_addr_mask;
> @@ -5091,7 +5091,7 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
>   	last_addr_mask = hugetlb_mask_last_page(h);
>   	address = start;
>   	for (; address < end; address += sz) {
> -		ptep = huge_pte_offset(mm, address, sz);
> +		ptep = hugetlb_walk(vma, address, sz);
>   		if (!ptep) {
>   			address |= last_addr_mask;
>   			continue;
> @@ -5404,7 +5404,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>   			mutex_lock(&hugetlb_fault_mutex_table[hash]);
>   			hugetlb_vma_lock_read(vma);
>   			spin_lock(ptl);
> -			ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
> +			ptep = hugetlb_walk(vma, haddr, huge_page_size(h));
>   			if (likely(ptep &&
>   				   pte_same(huge_ptep_get(ptep), pte)))
>   				goto retry_avoidcopy;
> @@ -5442,7 +5442,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>   	 * before the page tables are altered
>   	 */
>   	spin_lock(ptl);
> -	ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
> +	ptep = hugetlb_walk(vma, haddr, huge_page_size(h));
>   	if (likely(ptep && pte_same(huge_ptep_get(ptep), pte))) {
>   		/* Break COW or unshare */
>   		huge_ptep_clear_flush(vma, haddr, ptep);
> @@ -6227,7 +6227,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>   		return NULL;
>   
>   	hugetlb_vma_lock_read(vma);
> -	pte = huge_pte_offset(mm, haddr, huge_page_size(h));
> +	pte = hugetlb_walk(vma, haddr, huge_page_size(h));
>   	if (!pte)
>   		goto out_unlock;
>   
> @@ -6292,8 +6292,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   		 *
>   		 * Note that page table lock is not held when pte is null.
>   		 */
> -		pte = huge_pte_offset(mm, vaddr & huge_page_mask(h),
> -				      huge_page_size(h));
> +		pte = hugetlb_walk(vma, vaddr & huge_page_mask(h),
> +				   huge_page_size(h));
>   		if (pte)
>   			ptl = huge_pte_lock(h, mm, pte);
>   		absent = !pte || huge_pte_none(huge_ptep_get(pte));
> @@ -6479,7 +6479,7 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
>   	last_addr_mask = hugetlb_mask_last_page(h);
>   	for (; address < end; address += psize) {
>   		spinlock_t *ptl;
> -		ptep = huge_pte_offset(mm, address, psize);
> +		ptep = hugetlb_walk(vma, address, psize);
>   		if (!ptep) {
>   			address |= last_addr_mask;
>   			continue;
> @@ -6857,12 +6857,6 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
>   		*end = ALIGN(*end, PUD_SIZE);
>   }
>   
> -static bool __vma_shareable_flags_pmd(struct vm_area_struct *vma)
> -{
> -	return vma->vm_flags & (VM_MAYSHARE | VM_SHARED) &&
> -		vma->vm_private_data;
> -}
> -
>   void hugetlb_vma_lock_read(struct vm_area_struct *vma)
>   {
>   	if (__vma_shareable_flags_pmd(vma)) {
> @@ -7028,8 +7022,8 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
>   
>   		saddr = page_table_shareable(svma, vma, addr, idx);
>   		if (saddr) {
> -			spte = huge_pte_offset(svma->vm_mm, saddr,
> -					       vma_mmu_pagesize(svma));
> +			spte = hugetlb_walk(svma, saddr,
> +					    vma_mmu_pagesize(svma));
>   			if (spte) {
>   				get_page(virt_to_page(spte));
>   				break;
> @@ -7387,7 +7381,7 @@ void hugetlb_unshare_all_pmds(struct vm_area_struct *vma)
>   	hugetlb_vma_lock_write(vma);
>   	i_mmap_lock_write(vma->vm_file->f_mapping);
>   	for (address = start; address < end; address += PUD_SIZE) {
> -		ptep = huge_pte_offset(mm, address, sz);
> +		ptep = hugetlb_walk(vma, address, sz);
>   		if (!ptep)
>   			continue;
>   		ptl = huge_pte_lock(h, mm, ptep);
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 93e13fc17d3c..e97b2e23bd28 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -170,7 +170,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>   			return not_found(pvmw);
>   
>   		/* when pud is not present, pte will be NULL */
> -		pvmw->pte = huge_pte_offset(mm, pvmw->address, size);
> +		pvmw->pte = hugetlb_walk(vma, pvmw->address, size);
>   		if (!pvmw->pte)
>   			return false;
>   
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index d98564a7be57..cb23f8a15c13 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -305,13 +305,11 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
>   	hugetlb_vma_lock_read(vma);
>   	do {
>   		next = hugetlb_entry_end(h, addr, end);
> -		pte = huge_pte_offset(walk->mm, addr & hmask, sz);
> -
> +		pte = hugetlb_walk(vma, addr & hmask, sz);
>   		if (pte)
>   			err = ops->hugetlb_entry(pte, hmask, addr, next, walk);
>   		else if (ops->pte_hole)
>   			err = ops->pte_hole(addr, next, -1, walk);
> -
>   		if (err)
>   			break;
>   	} while (addr = next, addr != end);