On 12/8/20 7:34 PM, 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@xxxxxxxxxx>
>> 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.
>
/me nods
> 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?)
>
> 'refs' is number of tail pages within the compound, so 'ntails' or
> something
>
The usage of 'refs' seems to align with the rest of the GUP code. It's always referring to
tail pages and unpin case isn't any different IIUC.
I suppose we can always change that, but maybe better do that renaming in one shot as a
post cleanup?
>> {
>> - int refs = 1;
>> + int orig_refs = refs;
>>
>> page = compound_head(page);
>
> Caller should always do this
>
/me nods
>> @@ -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);
>> 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.
>
It appears to be case based on John's follow up comment.
>> +void unpin_user_page(struct page *page)
>> +{
>> + __unpin_user_page(page, 1);
>
> Thus this is
>
> __unpin_user_page(compound_head(page), 1);
>
Got it.
>> @@ -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
>> @@ -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);
> }
>
/me nods Let me attempt at that.
> And maybe you open code that iteration, but that basic idea to find a
> compound_head and ntails should be computational work performed.
>
I like the idea of a page range API alternative to unpin_user_pages(), but
improving current unpin_user_pages() would improve other unpin users too.
Perhaps the logic can be common, and the current unpin_user_pages() would have
the second iteration part, while the new (faster) API be based on computation.
> No reason not to fix set_page_dirty_lock() too while you are here.
>
OK.
> 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.
>
Yeap, let me do that.
> .. and I was just talking about this with Daniel Jordan and some other
> people at your company :)
>
:)
