Re: [PATCH v9 23/25] mm/gup: track FOLL_PIN pages

[John Hubbard <jhubbard@nvidia.com>]

On 12/11/19 3:28 AM, Jan Kara wrote:
...
> 
> The patch looks mostly good to me now. Just a few smaller comments below.
> 
>> Suggested-by: Jan Kara <jack@suse.cz>
>> Suggested-by: Jérôme Glisse <jglisse@redhat.com>
>> Reviewed-by: Jan Kara <jack@suse.cz>
>> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
>> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> 
> I think you inherited here the Reviewed-by tags from the "add flags" patch
> you've merged into this one but that's not really fair since this patch
> does much more... In particular I didn't give my Reviewed-by tag for this
> patch yet.

OK, I've removed those reviewed-by's. (I felt bad about dropping them, after
people had devoted time to reviewing, but I do see that it's wrong to imply
that they've reviewed this much much larger thing.)

...
> 
> I somewhat wonder about the asymmetry of try_grab_compound_head() vs
> try_grab_page() in the treatment of 'flags'. How costly would it be to make
> them symmetric (i.e., either set FOLL_GET for try_grab_compound_head()
> callers or make sure one of FOLL_GET, FOLL_PIN is set for try_grab_page())?
> 
> Because this difference looks like a subtle catch in the long run...

Done. It is only a modest code-level change, at least the way I've done it, which is
setting FOLL_GET for try_grab_compound_head(). In order to do that, I set
it at the top of the internal gup fast calling stacks, which is actually a good
design anyway: gup fast is logically doing FOLL_GET in all cases. So setting
the flag internally is accurate and consistent with the overall design.


> ...
> 
>> @@ -1522,8 +1536,8 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>>   skip_mlock:
>>   	page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
>>   	VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
>> -	if (flags & FOLL_GET)
>> -		get_page(page);
>> +	if (!try_grab_page(page, flags))
>> +		page = ERR_PTR(-EFAULT);
> 
> I think you need to also move the try_grab_page() earlier in the function.
> At this point the page may be marked as mlocked and you'd need to undo that
> in case try_grab_page() fails.


OK, I've moved it up, adding a "subpage" variable in order to make that work.

> 
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index ac65bb5e38ac..0aab6fe0072f 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -4356,7 +4356,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>>   same_page:
>>   		if (pages) {
>>   			pages[i] = mem_map_offset(page, pfn_offset);
>> -			get_page(pages[i]);
>> +			if (!try_grab_page(pages[i], flags)) {
>> +				spin_unlock(ptl);
>> +				remainder = 0;
>> +				err = -ENOMEM;
>> +				WARN_ON_ONCE(1);
>> +				break;
>> +			}
>>   		}
> 
> This function does a refcount overflow check early so that it doesn't have
> to do try_get_page() here. So that check can be now removed when you do
> try_grab_page() here anyway since that early check seems to be just a tiny
> optimization AFAICT.
> 
> 								Honza
> 

Yes. I've removed it, good spot.


thanks,
-- 
John Hubbard
NVIDIA