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Date:   Fri, 30 Nov 2018 21:11:17 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-aio@kvack.org, hch@lst.de
Subject: Re: [PATCH 24/27] block: implement bio helper to add iter kvec pages
 to bio
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On Fri, Nov 30, 2018 at 01:32:21PM -0700, Jens Axboe wrote:
> On 11/30/18 1:15 PM, Jens Axboe wrote:
> > On 11/30/18 12:21 PM, Al Viro wrote:
> >> On Fri, Nov 30, 2018 at 09:56:43AM -0700, Jens Axboe wrote:
> >>> For an ITER_KVEC, we can just iterate the iov and add the pages
> >>> to the bio directly.
> >>
> >>> +		page = virt_to_page(kv->iov_base);
> >>> +		size = bio_add_page(bio, page, kv->iov_len,
> >>> +					offset_in_page(kv->iov_base));
> >>
> >> Who said that you *can* do virt_to_page() on those?  E.g. vmalloc()'ed
> >> addresses are fine for ITER_KVEC, etc.
> > 
> > Then how do you set up a kvec based iter with memory you can safely
> > DMA to/from?
> 
> Would this make you happy:
> 
> if (!is_vmalloc_addr(kv->iov_base))
>         page = virt_to_page(kv->iov_base);
> else
>         page = vmalloc_to_page(kv->iov_base);

Free advice: don't ever let Linus see anything along those lines.  Results
tend to be colourful...