On 26.04.24 15:49, Pasha Tatashin wrote:
On Fri, Apr 26, 2024 at 2:42 AM David Hildenbrand <david@xxxxxxxxxx> 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I think the documentation is a bit outdated, because we now have page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
