On 7/28/21 5:08 PM, Dan Williams wrote:
> On Wed, Jul 28, 2021 at 8:56 AM Joao Martins <[email protected]>
> wrote:
>>
>> On 7/28/21 8:28 AM, Dan Williams wrote:
>>> On Wed, Jul 14, 2021 at 12:36 PM Joao Martins <[email protected]>
>>> wrote:
>>>>
>>>> + /*
>>>> + * With compound page geometry and when struct pages are stored in
>>>> ram
>>>> + * (!altmap) most tail pages are reused. Consequently, the amount
>>>> of
>>>> + * unique struct pages to initialize is a lot smaller that the
>>>> total
>>>> + * amount of struct pages being mapped.
>>>> + * See vmemmap_populate_compound_pages().
>>>> + */
>>>> + if (!altmap)
>>>> + nr_pages = min_t(unsigned long, nr_pages,
>>>
>>> What's the scenario where nr_pages is < 128? Shouldn't alignment
>>> already be guaranteed?
>>>
>> Oh yeah, that's right.
>>
>>>> + 2 * (PAGE_SIZE/sizeof(struct page)));
>>>
>>>
>>>> +
>>>> __SetPageHead(page);
>>>>
>>>> for (i = 1; i < nr_pages; i++) {
>>>> @@ -6657,7 +6669,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
>>>> continue;
>>>>
>>>> memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
>>>> - pfns_per_compound);
>>>> + altmap, pfns_per_compound);
>>>
>>> This feels odd, memmap_init_compound() doesn't really care about
>>> altmap, what do you think about explicitly calculating the parameters
>>> that memmap_init_compound() needs and passing them in?
>>>
>>> Not a strong requirement to change, but take another look at let me know.
>>>
>>
>> Yeah, memmap_init_compound() indeed doesn't care about @altmap itself -- but
>> a previous
>> comment was to abstract this away in memmap_init_compound() given the mix of
>> complexity in
>> memmap_init_zone_device() PAGE_SIZE geometry case and the compound case:
>>
>> https://lore.kernel.org/linux-mm/capcyv4gtsqfmuaax9cs63ovlkf-h4b_5fpienm9p9cqlzzt...@mail.gmail.com/
>>
>> Before this was called @ntails above and I hide that calculation in
>> memmap_init_compound().
>>
>> But I can move this back to the caller:
>>
>> memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
>> (!altmap ? 2 * (PAGE_SIZE/sizeof(struct page))) : pfns_per_compound);
>>
>> Or with another helper like:
>>
>> #define compound_nr_pages(__altmap, __nr_pages) \
>> (!__altmap ? 2 * (PAGE_SIZE/sizeof(struct page))) :
>> __nr_pages);
>>
>> memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
>> compound_nr_pages(altmap, pfns_per_compound));
>
> I like the helper, but I'd go further to make it a function with a
> comment that it is a paired / mild layering violation with explicit
> knowledge of how the sparse_vmemmap() internals handle compound pages
> in the presence of an altmap. I.e. if someone later goes to add altmap
> support, leave them a breadcrumb that they need to update both
> locations.
>
OK, got it.
