Re: [PATCH v2] mm/gup: fix gup_fast with dynamic page table folding

[John Hubbard <jhubbard@nvidia.com>]

On 9/11/20 1:36 PM, Vasily Gorbik wrote:
> Currently to make sure that every page table entry is read just once
> gup_fast walks perform READ_ONCE and pass pXd value down to the next
> gup_pXd_range function by value e.g.:
> 
> static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
>                           unsigned int flags, struct page **pages, int *nr)
> ...
>          pudp = pud_offset(&p4d, addr);
> 
> This function passes a reference on that local value copy to pXd_offset,
> and might get the very same pointer in return. This happens when the
> level is folded (on most arches), and that pointer should not be iterated.
> 
> On s390 due to the fact that each task might have different 5,4 or
> 3-level address translation and hence different levels folded the logic
> is more complex and non-iteratable pointer to a local copy leads to
> severe problems.
> 
> Here is an example of what happens with gup_fast on s390, for a task
> with 3-levels paging, crossing a 2 GB pud boundary:
> 
> // addr = 0x1007ffff000, end = 0x10080001000
> static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
>                           unsigned int flags, struct page **pages, int *nr)
> {
>          unsigned long next;
>          pud_t *pudp;
> 
>          // pud_offset returns &p4d itself (a pointer to a value on stack)
>          pudp = pud_offset(&p4d, addr);
>          do {
>                  // on second iteratation reading "random" stack value
>                  pud_t pud = READ_ONCE(*pudp);
> 
>                  // next = 0x10080000000, due to PUD_SIZE/MASK != PGDIR_SIZE/MASK on s390
>                  next = pud_addr_end(addr, end);
>                  ...
>          } while (pudp++, addr = next, addr != end); // pudp++ iterating over stack
> 
>          return 1;
> }
> 
> This happens since s390 moved to common gup code with
> commit d1874a0c2805 ("s390/mm: make the pxd_offset functions more robust")
> and commit 1a42010cdc26 ("s390/mm: convert to the generic
> get_user_pages_fast code"). s390 tried to mimic static level folding by
> changing pXd_offset primitives to always calculate top level page table
> offset in pgd_offset and just return the value passed when pXd_offset
> has to act as folded.
> 
> What is crucial for gup_fast and what has been overlooked is
> that PxD_SIZE/MASK and thus pXd_addr_end should also change
> correspondingly. And the latter is not possible with dynamic folding.
> 
> To fix the issue in addition to pXd values pass original
> pXdp pointers down to gup_pXd_range functions. And introduce
> pXd_offset_lockless helpers, which take an additional pXd
> entry value parameter. This has already been discussed in
> https://lkml.kernel.org/r/20190418100218.0a4afd51@mschwideX1
> 
> Cc: <stable@vger.kernel.org> # 5.2+
> Fixes: 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast code")
> Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
> Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
> Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
> ---

Looks cleaner than I'd dared hope for. :)

Reviewed-by: John Hubbard <jhubbard@nvidia.com>


thanks,
-- 
John Hubbard
NVIDIA

> v2: added brackets &pgd -> &(pgd)
> 
>   arch/s390/include/asm/pgtable.h | 42 +++++++++++++++++++++++----------
>   include/linux/pgtable.h         | 10 ++++++++
>   mm/gup.c                        | 18 +++++++-------
>   3 files changed, 49 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 7eb01a5459cd..b55561cc8786 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -1260,26 +1260,44 @@ static inline pgd_t *pgd_offset_raw(pgd_t *pgd, unsigned long address)
>   
>   #define pgd_offset(mm, address) pgd_offset_raw(READ_ONCE((mm)->pgd), address)
>   
> -static inline p4d_t *p4d_offset(pgd_t *pgd, unsigned long address)
> +static inline p4d_t *p4d_offset_lockless(pgd_t *pgdp, pgd_t pgd, unsigned long address)
>   {
> -	if ((pgd_val(*pgd) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R1)
> -		return (p4d_t *) pgd_deref(*pgd) + p4d_index(address);
> -	return (p4d_t *) pgd;
> +	if ((pgd_val(pgd) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R1)
> +		return (p4d_t *) pgd_deref(pgd) + p4d_index(address);
> +	return (p4d_t *) pgdp;
>   }
> +#define p4d_offset_lockless p4d_offset_lockless
>   
> -static inline pud_t *pud_offset(p4d_t *p4d, unsigned long address)
> +static inline p4d_t *p4d_offset(pgd_t *pgdp, unsigned long address)
>   {
> -	if ((p4d_val(*p4d) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R2)
> -		return (pud_t *) p4d_deref(*p4d) + pud_index(address);
> -	return (pud_t *) p4d;
> +	return p4d_offset_lockless(pgdp, *pgdp, address);
> +}
> +
> +static inline pud_t *pud_offset_lockless(p4d_t *p4dp, p4d_t p4d, unsigned long address)
> +{
> +	if ((p4d_val(p4d) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R2)
> +		return (pud_t *) p4d_deref(p4d) + pud_index(address);
> +	return (pud_t *) p4dp;
> +}
> +#define pud_offset_lockless pud_offset_lockless
> +
> +static inline pud_t *pud_offset(p4d_t *p4dp, unsigned long address)
> +{
> +	return pud_offset_lockless(p4dp, *p4dp, address);
>   }
>   #define pud_offset pud_offset
>   
> -static inline pmd_t *pmd_offset(pud_t *pud, unsigned long address)
> +static inline pmd_t *pmd_offset_lockless(pud_t *pudp, pud_t pud, unsigned long address)
> +{
> +	if ((pud_val(pud) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R3)
> +		return (pmd_t *) pud_deref(pud) + pmd_index(address);
> +	return (pmd_t *) pudp;
> +}
> +#define pmd_offset_lockless pmd_offset_lockless
> +
> +static inline pmd_t *pmd_offset(pud_t *pudp, unsigned long address)
>   {
> -	if ((pud_val(*pud) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R3)
> -		return (pmd_t *) pud_deref(*pud) + pmd_index(address);
> -	return (pmd_t *) pud;
> +	return pmd_offset_lockless(pudp, *pudp, address);
>   }
>   #define pmd_offset pmd_offset
>   
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index e8cbc2e795d5..90654cb63e9e 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1427,6 +1427,16 @@ typedef unsigned int pgtbl_mod_mask;
>   #define mm_pmd_folded(mm)	__is_defined(__PAGETABLE_PMD_FOLDED)
>   #endif
>   
> +#ifndef p4d_offset_lockless
> +#define p4d_offset_lockless(pgdp, pgd, address) p4d_offset(&(pgd), address)
> +#endif
> +#ifndef pud_offset_lockless
> +#define pud_offset_lockless(p4dp, p4d, address) pud_offset(&(p4d), address)
> +#endif
> +#ifndef pmd_offset_lockless
> +#define pmd_offset_lockless(pudp, pud, address) pmd_offset(&(pud), address)
> +#endif
> +
>   /*
>    * p?d_leaf() - true if this entry is a final mapping to a physical address.
>    * This differs from p?d_huge() by the fact that they are always available (if
> diff --git a/mm/gup.c b/mm/gup.c
> index e5739a1974d5..578bf5bd8bf8 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2485,13 +2485,13 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
>   	return 1;
>   }
>   
> -static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
> +static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned long end,
>   		unsigned int flags, struct page **pages, int *nr)
>   {
>   	unsigned long next;
>   	pmd_t *pmdp;
>   
> -	pmdp = pmd_offset(&pud, addr);
> +	pmdp = pmd_offset_lockless(pudp, pud, addr);
>   	do {
>   		pmd_t pmd = READ_ONCE(*pmdp);
>   
> @@ -2528,13 +2528,13 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
>   	return 1;
>   }
>   
> -static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
> +static int gup_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr, unsigned long end,
>   			 unsigned int flags, struct page **pages, int *nr)
>   {
>   	unsigned long next;
>   	pud_t *pudp;
>   
> -	pudp = pud_offset(&p4d, addr);
> +	pudp = pud_offset_lockless(p4dp, p4d, addr);
>   	do {
>   		pud_t pud = READ_ONCE(*pudp);
>   
> @@ -2549,20 +2549,20 @@ static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
>   			if (!gup_huge_pd(__hugepd(pud_val(pud)), addr,
>   					 PUD_SHIFT, next, flags, pages, nr))
>   				return 0;
> -		} else if (!gup_pmd_range(pud, addr, next, flags, pages, nr))
> +		} else if (!gup_pmd_range(pudp, pud, addr, next, flags, pages, nr))
>   			return 0;
>   	} while (pudp++, addr = next, addr != end);
>   
>   	return 1;
>   }
>   
> -static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end,
> +static int gup_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr, unsigned long end,
>   			 unsigned int flags, struct page **pages, int *nr)
>   {
>   	unsigned long next;
>   	p4d_t *p4dp;
>   
> -	p4dp = p4d_offset(&pgd, addr);
> +	p4dp = p4d_offset_lockless(pgdp, pgd, addr);
>   	do {
>   		p4d_t p4d = READ_ONCE(*p4dp);
>   
> @@ -2574,7 +2574,7 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end,
>   			if (!gup_huge_pd(__hugepd(p4d_val(p4d)), addr,
>   					 P4D_SHIFT, next, flags, pages, nr))
>   				return 0;
> -		} else if (!gup_pud_range(p4d, addr, next, flags, pages, nr))
> +		} else if (!gup_pud_range(p4dp, p4d, addr, next, flags, pages, nr))
>   			return 0;
>   	} while (p4dp++, addr = next, addr != end);
>   
> @@ -2602,7 +2602,7 @@ static void gup_pgd_range(unsigned long addr, unsigned long end,
>   			if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
>   					 PGDIR_SHIFT, next, flags, pages, nr))
>   				return;
> -		} else if (!gup_p4d_range(pgd, addr, next, flags, pages, nr))
> +		} else if (!gup_p4d_range(pgdp, pgd, addr, next, flags, pages, nr))
>   			return;
>   	} while (pgdp++, addr = next, addr != end);
>   }
>