[PATCH v3 14/23] iommu/pages: Move from struct page to struct ioptdesc and folio

[Baolu Lu]

On 2/26/25 21:51, Jason Gunthorpe wrote:
> On Wed, Feb 26, 2025 at 08:42:23PM +0800, Baolu Lu wrote:
>> On 2025/2/26 3:39, Jason Gunthorpe wrote:
>>> This brings the iommu page table allocator into the modern world of having
>>> its own private page descriptor and not re-using fields from struct page
>>> for its own purpose. It follows the basic pattern of struct ptdesc which
>>> did this transformation for the CPU page table allocator.
>>>
>>> Currently iommu-pages is pretty basic so this isn't a huge benefit,
>>> however I see a coming need for features that CPU allocator has, like sub
>>> PAGE_SIZE allocations, and RCU freeing. This provides the base
>>> infrastructure to implement those cleanly.
>> I understand that this is intended as the start point of having private
>> descriptors for folios allocated to iommu drivers. But I don't believe
>> this is currently the case after this patch, as the underlying memory
>> remains a struct folio. This patch merely uses an iommu-pages specific
>> structure pointer to reference it.
> Right now the mm provides 64 bytes of per-page memory that is a struct
> page.
> 
> You can call that 64 bytes a struct folio sometimes, and we have now
> been also calling those bytes a struct XXdesc like this patch does.
> 
> This is all a slow incremental evolution toward giving each user of
> the per-page memory its own unique type and understanding of what it
> needs while removing use of of the highly overloaded struct page.
> 
> Eventually Matthew wants to drop the 64 bytes down to 8 bytes and
> allocate the per-page memory directly. This would allow each user to
> use more/less memory depending on their need.
> 
> https://kernelnewbies.org/MatthewWilcox/Memdescs
> 
> When that happens the
> 
> 	folio = __folio_alloc_node(gfp | __GFP_ZERO, order, nid);
> 
> Will turn into something maybe more like:
> 
>     ioptdesc = memdesc_alloc_node(gfp, order, nid, sizeof(struct ioptdesc));
> 
> And then the folio word would disappear from this code.
> 
> Right now things are going down Matthew's list:
> 
> https://kernelnewbies.org/MatthewWilcox/Memdescs/Path
> 
> This series is part of "Remove page->lru uses"

Cool! Thank you for the explanation.

Thanks,
baolu