Re: [PATCH v2 5/5] mm: memory_hotplug: unify Huge/LRU/non-LRU movable folio isolation

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

On 2024/8/27 23:10, David Hildenbrand wrote:
> On 27.08.24 03:26, Kefeng Wang wrote:
>>
>>
>> On 2024/8/26 22:55, David Hildenbrand wrote:
>>> On 17.08.24 10:49, Kefeng Wang wrote:
>>>> Use the isolate_folio_to_list() to unify hugetlb/LRU/non-LRU
>>>> folio isolation, which cleanup code a bit and save a few calls
>>>> to compound_head().
>>>>
>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>> ---

...
>>> Hm, remind me why we are changing the hugetlb code to not take a
>>> reference here? It does look odd.
>>
>> Different from folio_isolate_lru(), isolate_hugetlb() will check folio
>> and try get a reference of folio after get hugetlb_lock, other hugetlb
>> operation protected by this big lock too, so no need to take a reference
>> here.
> 
> But this hugetlb-special casing looks quite ... special TBH.
> 
> Is there no way to avoid it?
> 
> I'd prefer something like the following, with a comment
> 
> if (hugetlb)
>      /*
>       * We also want to migrate hugetlb folios that span multiple
>           * memory blocks. So use whatever head page we identified.
>       */
>      folio = folio_try_get(folio);
> else
>      folio = folio_get_nontail_page(page);
> 
> if (!folio)
>      continue;
> 
> 
> And then just dropping the reference unconditionally.
> 
> Is there a problem with that? Optimizing for hugetlb references during 
> migration is not worth the trouble.
> 
> 
> But now I wonder, why we not simply unconditionally do a folio_try_get() 

Yes,  I use folio_get_nontail_page() and folio_put() unconditionally in v1,
this will skip tail page, not consistent with previous behavior for
hugetlb, so I change to current way, but for migration, use
folio_try_get()/folio_put() is enough since we always migrate the whole
folio, it will be more simple. I will push an additional fix to v3,
which just sent a couple of hours ago.

[1] 
https://lore.kernel.org/linux-mm/20240827114728.3212578-1-wangkefeng.wang@huawei.com/T/#t