Re: [PATCH v2 2/3] dma-buf: system_heap: Add pagepool support to system heap

[Daniel Mentz]

On Wed, Jan 27, 2021 at 9:10 PM John Stultz <john.stultz@xxxxxxxxxx> wrote:
>
> On Wed, Jan 27, 2021 at 12:21 PM Daniel Mentz <danielmentz@xxxxxxxxxx> wrote:
> >
> > On Fri, Jan 22, 2021 at 7:47 PM John Stultz <john.stultz@xxxxxxxxxx> wrote:
> > > +static int system_heap_clear_pages(struct page **pages, int num, pgprot_t pgprot)
> > > +{
> > > +       void *addr = vmap(pages, num, VM_MAP, pgprot);
> > > +
> > > +       if (!addr)
> > > +               return -ENOMEM;
> > > +       memset(addr, 0, PAGE_SIZE * num);
> > > +       vunmap(addr);
> > > +       return 0;
> > > +}
> >
> > I thought that vmap/vunmap are expensive, and I am wondering if
> > there's a faster way that avoids vmap.
> >
> > How about lifting this code from lib/iov_iter.c
> > static void memzero_page(struct page *page, size_t offset, size_t len)
> > {
> >         char *addr = kmap_atomic(page);
> >         memset(addr + offset, 0, len);
> >         kunmap_atomic(addr);
> > }
> >
> > Or what about lifting that code from the old ion_cma_heap.c
> >
> > if (PageHighMem(pages)) {
> >         unsigned long nr_clear_pages = nr_pages;
> >         struct page *page = pages;
> >
> >         while (nr_clear_pages > 0) {
> >                 void *vaddr = kmap_atomic(page);
> >
> >                 memset(vaddr, 0, PAGE_SIZE);
> >                 kunmap_atomic(vaddr);
> >                 page++;
> >                 nr_clear_pages--;
> >         }
> > } else {
> >         memset(page_address(pages), 0, size);
> > }
>
> Though, this last memset only works since CMA is contiguous, so it
> probably needs to always do the kmap_atomic for each page, right?

Yeah, but with the system heap page pool, some of these pages might be
64KB or 1MB large. kmap_atomic(page) just maps to page_address(page)
in most cases. I think iterating over all pages individually in this
manner might still be faster than using vmap.

>
> I'm still a little worried if this is right, as the current
> implementation with the vmap comes from the old ion_heap_sglist_zero
> logic, which similarly tries to batch the vmaps  32 pages at at time,
> but I'll give it a try.