Re: [PATCH v7 01/11] mm/mremap: Fix race between MOVE_PMD mremap and pageout

[Hugh Dickins <hughd@google.com>]

On Mon, 7 Jun 2021, Aneesh Kumar K.V wrote:

> CPU 1				CPU 2					CPU 3
> 
> mremap(old_addr, new_addr)      page_shrinker/try_to_unmap_one
> 
> mmap_write_lock_killable()
> 
> 				addr = old_addr
> 				lock(pte_ptl)
> lock(pmd_ptl)
> pmd = *old_pmd
> pmd_clear(old_pmd)
> flush_tlb_range(old_addr)
> 
> *new_pmd = pmd
> 									*new_addr = 10; and fills
> 									TLB with new addr
> 									and old pfn
> 
> unlock(pmd_ptl)
> 				ptep_clear_flush()
> 				old pfn is free.
> 									Stale TLB entry
> 
> Fix this race by holding pmd lock in pageout. This still doesn't handle the race
> between MOVE_PUD and pageout.
> 
> Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions")
> Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3S-huwbPw@mail.gmail.com
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

This seems very wrong to me, to require another level of locking in the
rmap lookup, just to fix some new pagetable games in mremap.

But Linus asked "Am I missing something?": neither of you have mentioned
mremap's take_rmap_locks(), so I hope that already meets your need.  And
if it needs to be called more often than before (see "need_rmap_locks"),
that's probably okay.

Hugh

> ---
>  include/linux/rmap.h |  9 ++++++---
>  mm/page_vma_mapped.c | 36 ++++++++++++++++++------------------
>  2 files changed, 24 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index def5c62c93b3..272ab0c2b60b 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -207,7 +207,8 @@ struct page_vma_mapped_walk {
>  	unsigned long address;
>  	pmd_t *pmd;
>  	pte_t *pte;
> -	spinlock_t *ptl;
> +	spinlock_t *pte_ptl;
> +	spinlock_t *pmd_ptl;
>  	unsigned int flags;
>  };
>  
> @@ -216,8 +217,10 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
>  	/* HugeTLB pte is set to the relevant page table entry without pte_mapped. */
>  	if (pvmw->pte && !PageHuge(pvmw->page))
>  		pte_unmap(pvmw->pte);
> -	if (pvmw->ptl)
> -		spin_unlock(pvmw->ptl);
> +	if (pvmw->pte_ptl)
> +		spin_unlock(pvmw->pte_ptl);
> +	if (pvmw->pmd_ptl)
> +		spin_unlock(pvmw->pmd_ptl);
>  }
>  
>  bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw);
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 2cf01d933f13..87a2c94c7e27 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -47,8 +47,10 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw)
>  				return false;
>  		}
>  	}
> -	pvmw->ptl = pte_lockptr(pvmw->vma->vm_mm, pvmw->pmd);
> -	spin_lock(pvmw->ptl);
> +	if (USE_SPLIT_PTE_PTLOCKS) {
> +		pvmw->pte_ptl = pte_lockptr(pvmw->vma->vm_mm, pvmw->pmd);
> +		spin_lock(pvmw->pte_ptl);
> +	}
>  	return true;
>  }
>  
> @@ -162,8 +164,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  		if (!pvmw->pte)
>  			return false;
>  
> -		pvmw->ptl = huge_pte_lockptr(page_hstate(page), mm, pvmw->pte);
> -		spin_lock(pvmw->ptl);
> +		pvmw->pte_ptl = huge_pte_lockptr(page_hstate(page), mm, pvmw->pte);
> +		spin_lock(pvmw->pte_ptl);
>  		if (!check_pte(pvmw))
>  			return not_found(pvmw);
>  		return true;
> @@ -179,6 +181,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  	if (!pud_present(*pud))
>  		return false;
>  	pvmw->pmd = pmd_offset(pud, pvmw->address);
> +	pvmw->pmd_ptl = pmd_lock(mm, pvmw->pmd);
>  	/*
>  	 * Make sure the pmd value isn't cached in a register by the
>  	 * compiler and used as a stale value after we've observed a
> @@ -186,7 +189,6 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  	 */
>  	pmde = READ_ONCE(*pvmw->pmd);
>  	if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) {
> -		pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>  		if (likely(pmd_trans_huge(*pvmw->pmd))) {
>  			if (pvmw->flags & PVMW_MIGRATION)
>  				return not_found(pvmw);
> @@ -206,14 +208,10 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  				}
>  			}
>  			return not_found(pvmw);
> -		} else {
> -			/* THP pmd was split under us: handle on pte level */
> -			spin_unlock(pvmw->ptl);
> -			pvmw->ptl = NULL;
>  		}
> -	} else if (!pmd_present(pmde)) {
> -		return false;
> -	}
> +	} else if (!pmd_present(pmde))
> +		return not_found(pvmw);
> +
>  	if (!map_pte(pvmw))
>  		goto next_pte;
>  	while (1) {
> @@ -233,19 +231,21 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  			/* Did we cross page table boundary? */
>  			if (pvmw->address % PMD_SIZE == 0) {
>  				pte_unmap(pvmw->pte);
> -				if (pvmw->ptl) {
> -					spin_unlock(pvmw->ptl);
> -					pvmw->ptl = NULL;
> +				if (pvmw->pte_ptl) {
> +					spin_unlock(pvmw->pte_ptl);
> +					pvmw->pte_ptl = NULL;
>  				}
> +				spin_unlock(pvmw->pmd_ptl);
> +				pvmw->pmd_ptl = NULL;
>  				goto restart;
>  			} else {
>  				pvmw->pte++;
>  			}
>  		} while (pte_none(*pvmw->pte));
>  
> -		if (!pvmw->ptl) {
> -			pvmw->ptl = pte_lockptr(mm, pvmw->pmd);
> -			spin_lock(pvmw->ptl);
> +		if (USE_SPLIT_PTE_PTLOCKS && !pvmw->pte_ptl) {
> +			pvmw->pte_ptl = pte_lockptr(mm, pvmw->pmd);
> +			spin_lock(pvmw->pte_ptl);
>  		}
>  	}
>  }
> -- 
> 2.31.1