linux-kernel - Re: [PATCH 3/4] tee: Remove call to get_kernel

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Date:   Sat, 1 Oct 2022 19:30:15 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Al Viro <viro@...iv.linux.org.uk>
CC:     Jens Wiklander <jens.wiklander@...aro.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        <op-tee@...ts.trustedfirmware.org>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>
Subject: Re: [PATCH 3/4] tee: Remove call to get_kernel_pages()

On Sun, Oct 02, 2022 at 01:46:41AM +0100, Al Viro wrote:
> On Sat, Oct 01, 2022 at 05:23:25PM -0700, ira.weiny@...el.com wrote:
> 
> >  	kiov = kcalloc(page_count, sizeof(*kiov), GFP_KERNEL);
> > @@ -38,12 +39,12 @@ static int shm_get_kernel_pages(unsigned long start, size_t page_count,
> >  	for (n = 0; n < page_count; n++) {
> >  		kiov[n].iov_base = (void *)(start + n * PAGE_SIZE);
> >  		kiov[n].iov_len = PAGE_SIZE;
> > +		pages[n] = virt_to_page(kiov[n].iov_base);
> > +		get_page(pages[n]);
> >  	}
> > -
> > -	rc = get_kernel_pages(kiov, page_count, 0, pages);
> >  	kfree(kiov);
> 
> IDGI.  The only thing in kiov[...] you are every reading is
> ->iov_base.  And you fetch it once, right after the assignment.

:-(  Good point.  Thanks for catching that.  I was too focused on just
replacing get_kernel_pages() with get_page() and I should have refactored
more.

> 
> Why bother with allocating the array at all?
> 		pages[n] = virt_to_page((void *)start + n * PAGE_SIZE);
> would do just as well, not to mention the fact that since you reject
> vmalloc and kmap, you might simply do
> 
> 	page = virt_to_page(start);
> 	for (int n = 0; n < page_count; n++)
> 		get_page(pages[n] = page + n);

I think I'd avoid the assignment in the parameter as I would miss that if I
came back and looked at this code later.

I'll get rid of the kiov in v2.

Sorry for not cleaning it up more and thanks for the review!

Ira