linux-kernel - Re: [RFC v2 0/3] iommu/intel: Free empty page tables on unmaps

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Date: Fri, 26 Apr 2024 16:39:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, rientjes@...gle.com, dwmw2@...radead.org,
 baolu.lu@...ux.intel.com, joro@...tes.org, will@...nel.org,
 robin.murphy@....com, iommu@...ts.linux.dev,
 Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC v2 0/3] iommu/intel: Free empty page tables on unmaps

On 26.04.24 15:49, Pasha Tatashin wrote:
> On Fri, Apr 26, 2024 at 2:42 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 26.04.24 05:43, Pasha Tatashin wrote:
>>> Changelog
>>> ================================================================
>>> v2: Use mapcount instead of refcount
>>>       Synchronized with IOMMU Observability changes.
>>> ================================================================
>>>
>>> This series frees empty page tables on unmaps. It intends to be a
>>> low overhead feature.
>>>
>>> The read-writer lock is used to synchronize page table, but most of
>>> time the lock is held is reader. It is held as a writer for short
>>> period of time when unmapping a page that is bigger than the current
>>> iova request. For all other cases this lock is read-only.
>>>
>>> page->mapcount is used in order to track number of entries at each page
>>> table.
>>
>> I'm wondering if this will conflict with page_type at some point? We're
>> already converting other page table users to ptdesc. CCing Willy.
> 
> Hi David,

Hi!

> 
> This contradicts with the following comment in mm_types.h:
>   * If your page will not be mapped to userspace, you can also use the four
>   * bytes in the mapcount union, but you must call
> page_mapcount_reset()
>   * before freeing it.

I think the documentation is a bit outdated, because we now have page 
types that are: "For pages that are never mapped to userspace"

which includes

#define PG_table

(we should update that comment, because we're now also using it for 
hugetlb that can be mapped to user space, which is fine.)

Right now, using page->_mapcount would likely still be fine, as long as 
you cannot end up creating a value that would resemble a type (e.g., 
PG_offline could be bad).

But staring at users of _mapcount and page_mapcount_reset() ... you'd be 
pretty much the only user of that.

mm/zsmalloc.c calls page_mapcount_reset(), and I am not completely sure 
why ... I can see it touch page->index but not page->_mapcount.


Hopefully Willy can comment.

-- 
Cheers,

David / dhildenb