On Thu, Oct 07, 2021 at 05:03:12PM +0200, Vlastimil Babka wrote: > On 10/5/21 19:51, Matthew Wilcox wrote: > > We're trying to tidy up the mess in struct page, and as part of removing > > slab from struct page, zsmalloc came on my radar because it's using some > > of slab's fields. The eventual endgame is to get struct page down to a > > single word which points to the "memory descriptor" (ie the current > > zspage). > > > > zsmalloc, like vmalloc, allocates order-0 pages. Unlike vmalloc, > > zsmalloc allows compaction. Currently (from the file): > > > > * Usage of struct page fields: > > * page->private: points to zspage > > * page->freelist(index): links together all component pages of a zspage > > * For the huge page, this is always 0, so we use this field > > * to store handle. > > * page->units: first object offset in a subpage of zspage > > * > > * Usage of struct page flags: > > * PG_private: identifies the first component page > > * PG_owner_priv_1: identifies the huge component page > > > > This isn't quite everything. For compaction, zsmalloc also uses > > page->mapping (set in __SetPageMovable()), PG_lock (to sync with > > compaction) and page->_refcount (compaction gets a refcount on the page). > > > > Since zsmalloc is so well-contained, I propose we completely stop > > using struct page in it, as we intend to do for the rest of the users > > of struct page. That is, the _only_ element of struct page we use is > > compound_head and it points to struct zspage. > > > > That means every single page allocated by zsmalloc is PageTail(). Also it > > I would be worried there is code, i.e. some pfn scanner that will see a > PageTail, lookup its compound_head() and order and use it to skip over the > rest of tail pages. Which would fail spectacularly if compound_head() > pointed somewhere else than to the same memmap array to a struct page. Yes, that's definitely a concern. What does work is the pfn scanner doing pfn |= (1 << page_order(page)) - 1; (because page_order(zspage) is 0, so this is a noop). It's something that will need to be audited before we do this.
