Re: [PATCH hmm 1/8] mm/hmm: add missing unmaps of the ptep during hmm_vma_handle_pte()

[Ralph Campbell <rcampbell@nvidia.com>]

On 3/11/20 11:34 AM, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@mellanox.com>
> 
> Many of the direct returns of error skipped doing the pte_unmap(). All non
> zero exit paths must unmap the pte.
> 
> The pte_unmap() is split unnaturally like this because some of the error
> exit paths trigger a sleep and must release the lock before sleeping.
> 
> Fixes: 992de9a8b751 ("mm/hmm: allow to mirror vma of a file on a DAX backed filesystem")
> Fixes: 53f5c3f489ec ("mm/hmm: factor out pte and pmd handling to simplify hmm_vma_walk_pmd()")
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

The changes look OK to me but one issue noted below.
In any case, you can add:
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>

> ---
>   mm/hmm.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 72e5a6d9a41756..35f85424176d14 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -325,6 +325,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>   		}
>   
>   		/* Report error for everything else */
> +		pte_unmap(ptep);
>   		*pfn = range->values[HMM_PFN_ERROR];
>   		return -EFAULT;
>   	} else {
> @@ -339,10 +340,13 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>   	if (pte_devmap(pte)) {
>   		hmm_vma_walk->pgmap = get_dev_pagemap(pte_pfn(pte),
>   					      hmm_vma_walk->pgmap);
> -		if (unlikely(!hmm_vma_walk->pgmap))
> +		if (unlikely(!hmm_vma_walk->pgmap)) {
> +			pte_unmap(ptep);
>   			return -EBUSY;
> +		}
>   	} else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
>   		if (!is_zero_pfn(pte_pfn(pte))) {
> +			pte_unmap(ptep);
>   			*pfn = range->values[HMM_PFN_SPECIAL];
>   			return -EFAULT;
>   		}
> @@ -437,7 +441,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>   
>   		r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, &pfns[i]);
>   		if (r) {
> -			/* hmm_vma_handle_pte() did unmap pte directory */
> +			/* hmm_vma_handle_pte() did pte_unmap() */
>   			hmm_vma_walk->last = addr;
>   			return r;
>   		}
> 

I think there is a case where hmm_vma_handle_pte() is called, a fault is requested,
pte_unmap() and hmm_vma_walk_hole_() are called, the latter returns zero (the fault
was handled OK), but now the page table is unmapped for successive loop iterations
and pte_unmap(ptep - 1) is called at the loop end.
Since this isn't an issue on x86_64 but is on x86_32, I can see how it could be missed.