Re: [PATCH v1 13/15] mm: handling Non-LRU pages returned by vm_normal_pages

Sierra Guiza, Alejandro (Alex)
2022-05-12T22:33:56Z

On 5/11/2022 1:50 PM, Jason Gunthorpe wrote:
> On Thu, May 05, 2022 at 04:34:36PM -0500, Alex Sierra wrote:
>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 76e3af9639d9..892c4cc54dc2 100644
>> +++ b/mm/memory.c
>> @@ -621,6 +621,13 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>> if (is_zero_pfn(pfn))
>> return NULL;
>> if (pte_devmap(pte))
>> +/*
>> + * NOTE: Technically this should goto check_pfn label. However, page->_mapcount
>> + * is never incremented for device pages that are mmap through DAX mechanism
>> + * using pmem driver mounted into ext4 filesystem. When these pages are unmap,
>> + * zap_pte_range is called and vm_normal_page return a valid page with
>> + * page_mapcount() = 0, before page_remove_rmap is called.
>> + */
>> return NULL;
> ? Where does this series cause device coherent to be returned?
In our case, device coherent pages could be obtained as a result of
migration(Patches 6/7 of 15), ending up mapped in CPU page tables. Later
on, these pages might need to be returned by get_user_pages or other
callers through vm_normal_pages. Our approach in this series, is to
handle device-coherent-managed pages returned by vm_normal_pages, inside
each caller. EX. device coherent pages don’t support LRU lists, NUMA
migration or THP.
>
> Wasn't the plan to not set pte_devmap() ?

amdgpu does not set pte_devmap for our DEVICE_COHERENT pages. DEVMAP
flags are set by drivers like virtio_fs or pmem, where
MEMORY_DEVICE_FS_DAX type is used.
This patch series deals with DEVICE_COHERENT pages. My understanding
was, that the DAX code and DEVICE_GENERIC would be fixed up later by
someone more familiar with it. Were you expecting that we'd fix the DAX
usage of pte_devmap flags in this patch series as well?

Regards,
Alex Sierra

>
> Jason