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
