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 <20210123034655.102813-2-john.stultz@linaro.org> <CAE2F3rA3a-MWBHPZhGP9dMhEUqMkkvm6wXYYh6LDZ1zF9CAyUg@mail.gmail.com>
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From:   John Stultz <john.stultz@linaro.org>
Date:   Wed, 27 Jan 2021 21:10:15 -0800
Message-ID: <CALAqxLWkTQwQ8Xqnc+PAMWUO_C84jaGzOLqKFqkuk3=AFNHD6g@mail.gmail.com>
Subject: Re: [PATCH v2 2/3] dma-buf: system_heap: Add pagepool support to
 system heap
To:     Daniel Mentz <danielmentz@google.com>
Cc:     lkml <linux-kernel@vger.kernel.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Liam Mark <lmark@codeaurora.org>,
        Chris Goldsworthy <cgoldswo@codeaurora.org>,
        Laura Abbott <labbott@kernel.org>,
        Brian Starkey <Brian.Starkey@arm.com>,
        Hridya Valsaraju <hridya@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Sandeep Patil <sspatil@google.com>,
        =?UTF-8?Q?=C3=98rjan_Eide?= <orjan.eide@arm.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Ezequiel Garcia <ezequiel@collabora.com>,
        Simon Ser <contact@emersion.fr>,
        James Jones <jajones@nvidia.com>,
        linux-media <linux-media@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>
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On Wed, Jan 27, 2021 at 12:21 PM Daniel Mentz <danielmentz@google.com> wrote:
>
> On Fri, Jan 22, 2021 at 7:47 PM John Stultz <john.stultz@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?

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