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Date: Mon, 22 May 2023 20:52:19 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Hugh Dickins <hughd@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Mike Kravetz <mike.kravetz@...cle.com>,
Mike Rapoport <rppt@...nel.org>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Matthew Wilcox <willy@...radead.org>,
David Hildenbrand <david@...hat.com>,
Suren Baghdasaryan <surenb@...gle.com>,
Qi Zheng <zhengqi.arch@...edance.com>,
Yang Shi <shy828301@...il.com>,
Mel Gorman <mgorman@...hsingularity.net>,
Peter Xu <peterx@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
Alistair Popple <apopple@...dia.com>,
Ralph Campbell <rcampbell@...dia.com>,
Ira Weiny <ira.weiny@...el.com>,
Steven Price <steven.price@....com>,
SeongJae Park <sj@...nel.org>,
Naoya Horiguchi <naoya.horiguchi@....com>,
Christophe Leroy <christophe.leroy@...roup.eu>,
Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Anshuman Khandual <anshuman.khandual@....com>,
Pasha Tatashin <pasha.tatashin@...een.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Minchan Kim <minchan@...nel.org>,
Christoph Hellwig <hch@...radead.org>,
Song Liu <song@...nel.org>,
Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 29/31] mm/memory: handle_pte_fault() use
pte_offset_map_nolock()
On 2023/5/22 13:26, Hugh Dickins wrote:
> handle_pte_fault() use pte_offset_map_nolock() to get the vmf.ptl which
> corresponds to vmf.pte, instead of pte_lockptr() being used later, when
> there's a chance that the pmd entry might have changed, perhaps to none,
> or to a huge pmd, with no split ptlock in its struct page.
>
> Remove its pmd_devmap_trans_unstable() call: pte_offset_map_nolock()
> will handle that case by failing. Update the "morph" comment above,
> looking forward to when shmem or file collapse to THP may not take
> mmap_lock for write (or not at all).
>
> do_numa_page() use the vmf->ptl from handle_pte_fault() at first, but
> refresh it when refreshing vmf->pte.
>
> do_swap_page()'s pte_unmap_same() (the thing that takes ptl to verify a
> two-part PAE orig_pte) use the vmf->ptl from handle_pte_fault() too; but
> do_swap_page() is also used by anon THP's __collapse_huge_page_swapin(),
> so adjust that to set vmf->ptl by pte_offset_map_nolock().
>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
> mm/khugepaged.c | 6 ++++--
> mm/memory.c | 38 +++++++++++++-------------------------
> 2 files changed, 17 insertions(+), 27 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 49cfa7cdfe93..c11db2e78e95 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1005,6 +1005,7 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> unsigned long address, end = haddr + (HPAGE_PMD_NR * PAGE_SIZE);
> int result;
> pte_t *pte = NULL;
> + spinlock_t *ptl;
>
> for (address = haddr; address < end; address += PAGE_SIZE) {
> struct vm_fault vmf = {
> @@ -1016,7 +1017,7 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> };
>
> if (!pte++) {
> - pte = pte_offset_map(pmd, address);
> + pte = pte_offset_map_nolock(mm, pmd, address, &ptl);
> if (!pte) {
> mmap_read_unlock(mm);
> result = SCAN_PMD_NULL;
> @@ -1024,11 +1025,12 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> }
> }
>
> - vmf.orig_pte = *pte;
> + vmf.orig_pte = ptep_get_lockless(pte);
> if (!is_swap_pte(vmf.orig_pte))
> continue;
>
> vmf.pte = pte;
> + vmf.ptl = ptl;
> ret = do_swap_page(&vmf);
> /* Which unmaps pte (after perhaps re-checking the entry) */
> pte = NULL;
> diff --git a/mm/memory.c b/mm/memory.c
> index c7b920291a72..4ec46eecefd3 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2786,10 +2786,9 @@ static inline int pte_unmap_same(struct vm_fault *vmf)
> int same = 1;
> #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPTION)
> if (sizeof(pte_t) > sizeof(unsigned long)) {
> - spinlock_t *ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> - spin_lock(ptl);
> + spin_lock(vmf->ptl);
> same = pte_same(*vmf->pte, vmf->orig_pte);
> - spin_unlock(ptl);
> + spin_unlock(vmf->ptl);
> }
> #endif
> pte_unmap(vmf->pte);
> @@ -4696,7 +4695,6 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
> * validation through pte_unmap_same(). It's of NUMA type but
> * the pfn may be screwed if the read is non atomic.
> */
> - vmf->ptl = pte_lockptr(vma->vm_mm, vmf->pmd);
> spin_lock(vmf->ptl);
> if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte))) {
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> @@ -4767,8 +4765,10 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
> flags |= TNF_MIGRATED;
> } else {
> flags |= TNF_MIGRATE_FAIL;
> - vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
> - spin_lock(vmf->ptl);
> + vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> + vmf->address, &vmf->ptl);
> + if (unlikely(!vmf->pte))
> + goto out;
> if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte))) {
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> goto out;
> @@ -4897,27 +4897,16 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
> vmf->pte = NULL;
> vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
> } else {
> - /*
> - * If a huge pmd materialized under us just retry later. Use
> - * pmd_trans_unstable() via pmd_devmap_trans_unstable() instead
> - * of pmd_trans_huge() to ensure the pmd didn't become
> - * pmd_trans_huge under us and then back to pmd_none, as a
> - * result of MADV_DONTNEED running immediately after a huge pmd
> - * fault in a different thread of this mm, in turn leading to a
> - * misleading pmd_trans_huge() retval. All we have to ensure is
> - * that it is a regular pmd that we can walk with
> - * pte_offset_map() and we can do that through an atomic read
> - * in C, which is what pmd_trans_unstable() provides.
> - */
> - if (pmd_devmap_trans_unstable(vmf->pmd))
> - return 0;
> /*
> * A regular pmd is established and it can't morph into a huge
> - * pmd from under us anymore at this point because we hold the
> - * mmap_lock read mode and khugepaged takes it in write mode.
> - * So now it's safe to run pte_offset_map().
> + * pmd by anon khugepaged, since that takes mmap_lock in write
> + * mode; but shmem or file collapse to THP could still morph
> + * it into a huge pmd: just retry later if so.
> */
> - vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
> + vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
> + vmf->address, &vmf->ptl);
> + if (unlikely(!vmf->pte))
> + return 0;
Just jump to the retry label below?
diff --git a/mm/memory.c b/mm/memory.c
index 63632a5eafc1..2e712fe6f4be 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4897,7 +4897,8 @@ static vm_fault_t handle_pte_fault(struct vm_fault
*vmf)
{
pte_t entry;
- if (unlikely(pmd_none(*vmf->pmd))) {
+retry:
+ if (unlikely(pmd_none(READ_ONCE(*vmf->pmd)))) {
/*
* Leave __pte_alloc() until later: because
vm_ops->fault may
* want to allocate huge page, and if we expose page table
> vmf->orig_pte = ptep_get_lockless(vmf->pte);
> vmf->flags |= FAULT_FLAG_ORIG_PTE_VALID;
>
> @@ -4936,7 +4925,6 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
> if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
> return do_numa_page(vmf);
>
> - vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> spin_lock(vmf->ptl);
> entry = vmf->orig_pte;
> if (unlikely(!pte_same(*vmf->pte, entry))) {
--
Thanks,
Qi
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