Re: [PATCH RFC 7/9] mm/gup: Decrement head page once for group of subpages

[John Hubbard <jhubbard@nvidia.com>]

On 12/8/20 11:34 AM, Jason Gunthorpe wrote:
> On Tue, Dec 08, 2020 at 05:28:59PM +0000, Joao Martins wrote:
>> Rather than decrementing the ref count one by one, we
>> walk the page array and checking which belong to the same
>> compound_head. Later on we decrement the calculated amount
>> of references in a single write to the head page.
>>
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>>   mm/gup.c | 41 ++++++++++++++++++++++++++++++++---------
>>   1 file changed, 32 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 194e6981eb03..3a9a7229f418 100644
>> +++ b/mm/gup.c
>> @@ -212,6 +212,18 @@ static bool __unpin_devmap_managed_user_page(struct page *page)
>>   }
>>   #endif /* CONFIG_DEV_PAGEMAP_OPS */
>>   
>> +static int record_refs(struct page **pages, int npages)
>> +{
>> +	struct page *head = compound_head(pages[0]);
>> +	int refs = 1, index;
>> +
>> +	for (index = 1; index < npages; index++, refs++)
>> +		if (compound_head(pages[index]) != head)
>> +			break;
>> +
>> +	return refs;
>> +}
>> +
>>   /**
>>    * unpin_user_page() - release a dma-pinned page
>>    * @page:            pointer to page to be released
>> @@ -221,9 +233,9 @@ static bool __unpin_devmap_managed_user_page(struct page *page)
>>    * that such pages can be separately tracked and uniquely handled. In
>>    * particular, interactions with RDMA and filesystems need special handling.
>>    */
>> -void unpin_user_page(struct page *page)
>> +static void __unpin_user_page(struct page *page, int refs)
> 
> Refs should be unsigned everywhere.

That's fine (although, see my comments in the previous patch for
pitfalls). But it should be a preparatory patch, in order to avoid
clouding up this one and your others as well.


> 
> I suggest using clear language 'page' here should always be a compound
> head called 'head' (or do we have another common variable name for
> this?)
> 

Agreed. Matthew's struct folio upgrade will allow us to really make
things clear in a typesafe way, but meanwhile, it's probably good to use
one of the following patterns:

page = compound_head(page); // at the very beginning of a routine

or

do_things_to_this_single_page(page);

head = compound_head(page);
do_things_to_this_compound_page(head);


> 'refs' is number of tail pages within the compound, so 'ntails' or
> something
> 

I think it's OK to leave it as "refs", because within gup.c, refs has
a very particular meaning. But if you change to ntails or something, I'd
want to see a complete change: no leftovers of refs that are really ntails.

So far I'd rather leave it as refs, but it's not a big deal either way.

>>   {
>> -	int refs = 1;
>> +	int orig_refs = refs;
>>   
>>   	page = compound_head(page);
> 
> Caller should always do this
> 
>> @@ -237,14 +249,19 @@ void unpin_user_page(struct page *page)
>>   		return;
>>   
>>   	if (hpage_pincount_available(page))
>> -		hpage_pincount_sub(page, 1);
>> +		hpage_pincount_sub(page, refs);

Maybe a nice touch would be to pass in orig_refs, because there
is no intention to use a possibly modified refs. So:

		hpage_pincount_sub(page, orig_refs);

...obviously a fine point, I realize. :)

>>   	else
>> -		refs = GUP_PIN_COUNTING_BIAS;
>> +		refs *= GUP_PIN_COUNTING_BIAS;
>>   
>>   	if (page_ref_sub_and_test(page, refs))
>>   		__put_page(page);
>>   
>> -	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
>> +	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, orig_refs);
>> +}
> 
> And really this should be placed directly after
> try_grab_compound_head() and be given a similar name
> 'unpin_compound_head()'. Even better would be to split the FOLL_PIN
> part into a function so there was a clear logical pairing.
> 
> And reviewing it like that I want to ask if this unpin sequence is in
> the right order.. I would expect it to be the reverse order of the get
> 
> John?
> 
> Is it safe to call mod_node_page_state() after releasing the refcount?
> This could race with hot-unplugging the struct pages so I think it is
> wrong.

Yes, I think you are right! I wasn't in a hot unplug state of mind when I
thought about the ordering there, but I should have been. :)

> 
>> +void unpin_user_page(struct page *page)
>> +{
>> +	__unpin_user_page(page, 1);
> 
> Thus this is
> 
> 	__unpin_user_page(compound_head(page), 1);
> 
>> @@ -274,6 +291,7 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
>>   				 bool make_dirty)
>>   {
>>   	unsigned long index;
>> +	int refs = 1;
>>   
>>   	/*
>>   	 * TODO: this can be optimized for huge pages: if a series of pages is

I think you can delete this TODO block now, and the one in unpin_user_pages_dirty_lock(),
as a result of these changes.

>> @@ -286,8 +304,9 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
>>   		return;
>>   	}
>>   
>> -	for (index = 0; index < npages; index++) {
>> +	for (index = 0; index < npages; index += refs) {
>>   		struct page *page = compound_head(pages[index]);
>> +
> 
> I think this is really hard to read, it should end up as some:
> 
> for_each_compond_head(page_list, page_list_len, &head, &ntails) {
>         		if (!PageDirty(head))
> 			set_page_dirty_lock(head, ntails);
> 		unpin_user_page(head, ntails);
> }
> 
> And maybe you open code that iteration, but that basic idea to find a
> compound_head and ntails should be computational work performed.
> 
> No reason not to fix set_page_dirty_lock() too while you are here.

Eh? What's wrong with set_page_dirty_lock() ?

> 
> Also, this patch and the next can be completely independent of the
> rest of the series, it is valuable regardless of the other tricks. You
> can split them and progress them independently.
> 
> .. and I was just talking about this with Daniel Jordan and some other
> people at your company :)
> 
> Thanks,
> Jason
> 

thanks,
-- 
John Hubbard
NVIDIA
_______________________________________________
Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org
To unsubscribe send an email to linux-nvdimm-leave@lists.01.org