On Mon, Apr 13, 2026 at 11:05:40AM +0200, David Hildenbrand (Arm) wrote: > On 4/13/26 00:50, Michael S. Tsirkin wrote: > > The previous patch skips zeroing in post_alloc_hook() when > > __GFP_ZERO is used. However, several page allocation paths > > zero pages via folio_zero_user() or clear_user_highpage() after > > allocation, not via __GFP_ZERO. > > > > Add __GFP_PREZEROED gfp flag that tells post_alloc_hook() to > > preserve the MAGIC_PAGE_ZEROED sentinel in page->private so the > > caller can detect pre-zeroed pages and skip its own zeroing. > > Add folio_test_clear_prezeroed() helper to check and clear > > the sentinel. > > I really don't like __GFP_PREZEROED, and wonder how we can avoid it. > > > What you want is, allocate a folio (well, actually a page that becomes > a folio) and know whether zeroing for that folio (once we establish it > from a page) is still required. > > Or you just allocate a folio, specify GFP_ZERO, and let the folio > allocation code deal with that. > > > I think we have two options: > > (1) Use an indication that can be sticky for callers that do not care. > > Assuming we would use a page flag that is only ever used on folios, all > we'd have to do is make sure that we clear the flag once we convert > the to a folio. > > For example, PG_dropbehind is only ever set on folios in the pagecache. > > Paths that allocate folios would have to clear the flag. For non-hugetlb > folios that happens through page_rmappable_folio(). > > I'm not super-happy about that, but it would be doable. > > > (2) Use a dedicated allocation interface for user pages in the buddy. > > I hate the whole user_alloc_needs_zeroing()+folio_zero_user() handling. > > It shouldn't exist. We should just be passing GFP_ZERO and let the buddy > handle > all that. > > > For example, vma_alloc_folio() already gets passed the address in. > > Pass the address from vma_alloc_folio_noprof()->folio_alloc_noprof(), and let > folio_alloc_noprof() use a buddy interface that can handle it. > > Imagine if we had a alloc_user_pages_noprof() that consumes an address. It > could just > do what folio_zero_user() does, and only if really required. > > The whole user_alloc_needs_zeroing() could go away and you could just handle > the > pre-zeroed optimization internally. > > -- > Cheers, > > David
I admit I only vaguely understand the core mm refactoring you are suggesting. -- MST
