Re: [PATCH] mm/gup.c: Refactor check_and_migrate_movable_pages()

[Alistair Popple <apopple@nvidia.com>]

Jason Gunthorpe <jgg@nvidia.com> writes:

> On Thu, Aug 04, 2022 at 01:22:41PM +1000, Alistair Popple wrote:
>> When pinning pages with FOLL_LONGTERM check_and_migrate_movable_pages()
>> is called to migrate pages out of zones which should not contain any
>> longterm pinned pages.
>>
>> When migration succeeds all pages will have been unpinned so pinning
>> needs to be retried. Migration can also fail, in which case the pages
>> will also have been unpinned but the operation should not be retried. If
>> all pages are in the correct zone nothing will be unpinned and no retry
>> is required.
>>
>> The logic in check_and_migrate_movable_pages() tracks unnecessary state
>> and the return codes for each case are difficult to follow. Refactor the
>> code to clean this up. No behaviour change is intended.
>>
>> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>> Cc: "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>
>> Cc: Chaitanya Kulkarni <kch@nvidia.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Felix Kuehling <Felix.Kuehling@amd.com>
>> Cc: Jason Gunthorpe <jgg@nvidia.com>
>> Cc: John Hubbard <jhubbard@nvidia.com>
>> Cc: Logan Gunthorpe <logang@deltatee.com>
>> Cc: Miaohe Lin <linmiaohe@huawei.com>
>> Cc: Muchun Song <songmuchun@bytedance.com>
>> Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
>> Cc: Ralph Campbell <rcampbell@nvidia.com>
>>
>> ---
>>
>> Originally posted as "mm/gup.c: Simplify and fix
>> check_and_migrate_movable_pages() return codes"[1].
>>
>> Changes from that version:
>>
>>  - Restore the original isolation failure behaviour and don't fail the
>>    pup. Instead retry indefinitely.
>>  - Unpin all pages on retry or failure rather than just failure.
>>
>> Jason - I dropped your Reviewed-by. I had to remove the changes to make
>> error handling follow convention as we need to always unpin the pages.
>> We also need the list_empty() checks because we may or may not have
>> pages in the list if we found coherent pages. So there isn't much I
>> could see to simplify, but let me know if you spot some.
>
> I don't quite understand this, if the point is to loop on the LRU
> indefinately, why not just code that? Why do we need to go around the
> big loop?

I assume by "big loop" you mean calling
check_and_migrate_movable_pages() again? We have to do that after the
migration anyway. Looping on the LRU indefinitely inside
check_and_migrate_movable_pages() doesn't remove that requirement so I'm
not convinced it would simplify things much.

It could also lead to deadlock because we may already have other pages
isolated from the LRU.

 - Alistair

> Jason