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Subject: Re: [PATCH 13/31] mm/hmm: retry if pte_offset_map() fails
Date:   Tue, 23 May 2023 12:39:54 +1000
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Qi Zheng <qi.zheng@linux.dev> writes:

> On 2023/5/22 13:05, Hugh Dickins wrote:
>> hmm_vma_walk_pmd() is called through mm_walk, but already has a goto
>> again loop of its own, so take part in that if pte_offset_map() fails.
>> Signed-off-by: Hugh Dickins <hughd@google.com>
>> ---
>>   mm/hmm.c | 2 ++
>>   1 file changed, 2 insertions(+)
>> diff --git a/mm/hmm.c b/mm/hmm.c
>> index e23043345615..b1a9159d7c92 100644
>> --- a/mm/hmm.c
>> +++ b/mm/hmm.c
>> @@ -381,6 +381,8 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>>   	}
>>     	ptep = pte_offset_map(pmdp, addr);
>> +	if (!ptep)
>> +		goto again;
>>   	for (; addr < end; addr += PAGE_SIZE, ptep++, hmm_pfns++) {
>>   		int r;
>>   
>
> I haven't read the entire patch set yet, but taking a note here.
> The hmm_vma_handle_pte() will unmap pte and then call
> migration_entry_wait() to remap pte, so this may fail, we need to
> handle this case like below:

I don't see a problem here. Sure, hmm_vma_handle_pte() might return
-EBUSY but that will get returned up to hmm_range_fault() which will
retry the whole thing again and presumably fail when looking at the PMD.

> diff --git a/mm/hmm.c b/mm/hmm.c
> index 6a151c09de5e..eb726ff0981c 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -276,7 +276,8 @@ static int hmm_vma_handle_pte(struct mm_walk
> *walk, unsigned long addr,
>                 if (is_migration_entry(entry)) {
>                         pte_unmap(ptep);
>                         hmm_vma_walk->last = addr;
> -                       migration_entry_wait(walk->mm, pmdp, addr);
> +                       if (!migration_entry_wait(walk->mm, pmdp, addr))
> +                               return -EAGAIN;
>                         return -EBUSY;
>                 }
>
> @@ -386,6 +387,8 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>
>                 r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep,
>                 hmm_pfns);
>                 if (r) {
> +                       if (r == -EAGAIN)
> +                               goto again;
>                         /* hmm_vma_handle_pte() did pte_unmap() */
>                         return r;
>                 }
>
> Of course, the migration_entry_wait() also needs to be modified.