On Fri, Apr 26, 2024 at 04:39:05PM +0200, David Hildenbrand wrote: > 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 I think the documentation is a bit outdated, because we now have page types 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. I feel like I have to say "no" to Pasha far too often ;-( Agreed the documentation is out of date. I think there's a lot of space in the struct page that can be used. These are iommu page tables, not cpu page tables, so things are a bit different for them. But should they be converted to use ptdesc? Maybe! I'd suggest putting this into the union with pt_mm and pt_frag_refcount. I think it could even go in the union with pt_list, but I think I'd rather see it in the pt_mm union.
