Re: [PATCH] dm-crypt: allocate compound pages if possible

[Mikulas Patocka <mpatocka@redhat.com>]

On Thu, 16 Feb 2023, Matthew Wilcox wrote:

> > -		len = (remaining_size > PAGE_SIZE) ? PAGE_SIZE : remaining_size;
> > -
> > -		bio_add_page(clone, page, len, 0);
> > +have_pages:
> > +		page->compound_order = order;
> 
> No.  You'll corrupt the next page if page is order-0, which it is if it
> came from the mempool.  Also we've deleted page->compound_order in -next
> so you can't make this mistake.  Using __GFP_COMP will set this field
> for you, so you can just drop this line.

OK

> > -		remaining_size -= len;
> > +		for (o = 0; o < 1U << order; o++) {
> > +			unsigned len = min((unsigned)PAGE_SIZE, remaining_size);
> > +			bio_add_page(clone, page, len, 0);
> > +			remaining_size -= len;
> > +			page++;
> 
> You can add multiple pages at once, whether they're compound or not.  So
> replace this entire loop with:
> 
> 		bio_add_page(clone, page, remaining_size, 0);

This should be min((unsigned)PAGE_SIZE << order, remaining_size), because 
we may allocate less than remaining_size.

> > @@ -1711,10 +1732,23 @@ static void crypt_free_buffer_pages(stru
> >  {
> >  	struct bio_vec *bv;
> >  	struct bvec_iter_all iter_all;
> > +	unsigned skip_entries = 0;
> >  
> >  	bio_for_each_segment_all(bv, clone, iter_all) {
> > -		BUG_ON(!bv->bv_page);
> > -		mempool_free(bv->bv_page, &cc->page_pool);
> > +		unsigned order;
> > +		struct page *page = bv->bv_page;
> > +		BUG_ON(!page);
> > +		if (skip_entries) {
> > +			skip_entries--;
> > +			continue;
> > +		}
> > +		order = page->compound_order;
> > +		if (order) {
> > +			__free_pages(page, order);
> > +			skip_entries = (1U << order) - 1;
> > +		} else {
> > +			mempool_free(page, &cc->page_pool);
> > +		}
> 
> You can simplify this by using the folio code.
> 
> 	struct folio_iter fi;
> 
> 	bio_for_each_folio_all(fi, bio) {
> 		if (folio_test_large(folio))
> 			folio_put(folio);
> 		else
> 			mempool_free(&folio->page, &cc->page_pool);
> 	}

OK. I'm sending version 2 of the patch.

> (further work would actually convert this driver to use folios instead
> of pages)

Mikulas