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

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Date:   Wed, 27 Jan 2021 21:10:15 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Daniel Mentz <danielmentz@...gle.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Chris Goldsworthy <cgoldswo@...eaurora.org>,
        Laura Abbott <labbott@...nel.org>,
        Brian Starkey <Brian.Starkey@....com>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Ørjan Eide <orjan.eide@....com>,
        Robin Murphy <robin.murphy@....com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Simon Ser <contact@...rsion.fr>,
        James Jones <jajones@...dia.com>,
        linux-media <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v2 2/3] dma-buf: system_heap: Add pagepool support to
 system heap

On Wed, Jan 27, 2021 at 12:21 PM Daniel Mentz <danielmentz@...gle.com> wrote:
>
> On Fri, Jan 22, 2021 at 7:47 PM John Stultz <john.stultz@...aro.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?

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