Re: [PATCH 2/2] mm: support poison recovery from copy_present_page()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/9/10 10:19, Miaohe Lin wrote:
> On 2024/9/6 10:42, Kefeng Wang wrote:
>> Similar to other poison recovery, use copy_mc_user_highpage() to
>> avoid potentially kernel panic during copy page in copy_present_page()
>> from fork, once copy failed due to hwpoison in source page, we need
>> to break out of copy in copy_pte_range() and release prealloc folio,
>> so copy_mc_user_highpage() is moved ahead before set *prealloc to NULL.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   mm/memory.c | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index d310c073a1b3..6e7b78e49d1a 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -926,8 +926,11 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
>>   	 * We have a prealloc page, all good!  Take it
>>   	 * over and copy the page & arm it.
>>   	 */
>> +
>> +	if (copy_mc_user_highpage(&new_folio->page, page, addr, src_vma))
>> +		return -EHWPOISON;
>> +
>>   	*prealloc = NULL;
>> -	copy_user_highpage(&new_folio->page, page, addr, src_vma);
>>   	__folio_mark_uptodate(new_folio);
>>   	folio_add_new_anon_rmap(new_folio, dst_vma, addr, RMAP_EXCLUSIVE);
>>   	folio_add_lru_vma(new_folio, dst_vma);
>> @@ -1166,8 +1169,9 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>>   		/*
>>   		 * If we need a pre-allocated page for this pte, drop the
>>   		 * locks, allocate, and try again.
>> +		 * If copy failed due to hwpoison in source page, break out.
>>   		 */
>> -		if (unlikely(ret == -EAGAIN))
>> +		if (unlikely(ret == -EAGAIN || ret == -EHWPOISON))
> 
> Will it be better to put checking ret against -EHWPOISON in a new line? -EAGAIN case will enter the
> loop again but -EHWPOISON case never does.

Maybe not a newline since we will recheck the ret in the below and 
return directly from copy_present_page().

> 
>>   			break;
>>   		if (unlikely(prealloc)) {
>>   			/*
>> @@ -1197,7 +1201,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>>   			goto out;
>>   		}
>>   		entry.val = 0;
>> -	} else if (ret == -EBUSY) {
>> +	} else if (ret == -EBUSY || unlikely(ret == -EHWPOISON)) {
> 
> The caller of copy_pte_range() always set errno to -ENOMEM. So fork will failed with ENOMEM even if the real
> cause is failed to copy due to hwpoison in source page. It's a pity.

Yes, it's not just the new -EHWPOISON, all other errnos(ENOENT/EIO) will 
be ignored and return -ENOMEM instead.

> 
> Thanks.
> .