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?
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.
thanks
-john
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