On Wed, Jan 27, 2021 at 9:10 PM John Stultz <john.stultz at linaro.org> wrote:
>
> On Wed, Jan 27, 2021 at 12:21 PM Daniel Mentz <danielmentz at google.com> wrote:
> >
> > On Fri, Jan 22, 2021 at 7:47 PM John Stultz <john.stultz at linaro.org> 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.