Re: [PATCH hmm 7/8] mm/hmm: return -EFAULT when setting HMM_PFN_ERROR on requested valid pages

[Ralph Campbell <rcampbell@nvidia.com>]

On 3/11/20 11:35 AM, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@mellanox.com>
> 
> hmm_range_fault() should never return 0 if the caller requested a valid
> page, but the pfns output for that page would be HMM_PFN_ERROR.
> 
> hmm_pte_need_fault() must always be called before setting HMM_PFN_ERROR to
> detect if the page is in faulting mode or not.
> 
> Fix two cases in hmm_vma_walk_pmd() and reorganize some of the duplicated
> code.
> 
> Fixes: d08faca018c4 ("mm/hmm: properly handle migration pmd")
> Fixes: da4c3c735ea4 ("mm/hmm/mirror: helper to snapshot CPU page table")
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>

> ---
>   mm/hmm.c | 38 +++++++++++++++++++++-----------------
>   1 file changed, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index bf676cfef3e8ee..f61fddf2ef6505 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -363,8 +363,10 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>   {
>   	struct hmm_vma_walk *hmm_vma_walk = walk->private;
>   	struct hmm_range *range = hmm_vma_walk->range;
> -	uint64_t *pfns = range->pfns;
> -	unsigned long addr = start, i;
> +	uint64_t *pfns = &range->pfns[(start - range->start) >> PAGE_SHIFT];
> +	unsigned long npages = (end - start) >> PAGE_SHIFT;
> +	unsigned long addr = start;
> +	bool fault, write_fault;
>   	pte_t *ptep;
>   	pmd_t pmd;
>   
> @@ -374,14 +376,6 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>   		return hmm_vma_walk_hole(start, end, -1, walk);
>   
>   	if (thp_migration_supported() && is_pmd_migration_entry(pmd)) {
> -		bool fault, write_fault;
> -		unsigned long npages;
> -		uint64_t *pfns;
> -
> -		i = (addr - range->start) >> PAGE_SHIFT;
> -		npages = (end - addr) >> PAGE_SHIFT;
> -		pfns = &range->pfns[i];
> -
>   		hmm_range_need_fault(hmm_vma_walk, pfns, npages,
>   				     0, &fault, &write_fault);
>   		if (fault || write_fault) {
> @@ -390,8 +384,15 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>   			return -EBUSY;
>   		}
>   		return hmm_pfns_fill(start, end, range, HMM_PFN_NONE);
> -	} else if (!pmd_present(pmd))
> +	}
> +
> +	if (!pmd_present(pmd)) {
> +		hmm_range_need_fault(hmm_vma_walk, pfns, npages, 0, &fault,
> +				     &write_fault);
> +		if (fault || write_fault)
> +			return -EFAULT;
>   		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);

Shouldn't this fill with HMM_PFN_NONE instead of HMM_PFN_ERROR?
Otherwise, when a THP is swapped out, you will get a different
value than if a PTE is swapped out and you are prefetching/snapshotting.

> +	}
>   
>   	if (pmd_devmap(pmd) || pmd_trans_huge(pmd)) {
>   		/*
> @@ -408,8 +409,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>   		if (!pmd_devmap(pmd) && !pmd_trans_huge(pmd))
>   			goto again;
>   
> -		i = (addr - range->start) >> PAGE_SHIFT;
> -		return hmm_vma_handle_pmd(walk, addr, end, &pfns[i], pmd);
> +		return hmm_vma_handle_pmd(walk, addr, end, pfns, pmd);
>   	}
>   
>   	/*
> @@ -418,15 +418,19 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>   	 * entry pointing to pte directory or it is a bad pmd that will not
>   	 * recover.
>   	 */
> -	if (pmd_bad(pmd))
> +	if (pmd_bad(pmd)) {
> +		hmm_range_need_fault(hmm_vma_walk, pfns, npages, 0, &fault,
> +				     &write_fault);
> +		if (fault || write_fault)
> +			return -EFAULT;
>   		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
> +	}
>   
>   	ptep = pte_offset_map(pmdp, addr);
> -	i = (addr - range->start) >> PAGE_SHIFT;
> -	for (; addr < end; addr += PAGE_SIZE, ptep++, i++) {
> +	for (; addr < end; addr += PAGE_SIZE, ptep++, pfns++) {
>   		int r;
>   
> -		r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, &pfns[i]);
> +		r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, pfns);
>   		if (r) {
>   			/* hmm_vma_handle_pte() did pte_unmap() */
>   			hmm_vma_walk->last = addr;
> 
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