Messages in this thread | | | Date | Fri, 26 Apr 2024 16:39:05 +0200 | Subject | Re: [RFC v2 0/3] iommu/intel: Free empty page tables on unmaps | From | David Hildenbrand <> |
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On 26.04.24 15:49, Pasha Tatashin wrote: > On Fri, Apr 26, 2024 at 2:42 AM David Hildenbrand <david@redhat.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I think the documentation is a bit outdated, because we now have page types that are: "For pages that are never mapped to userspace"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
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