linux-kernel - Re: [PATCH 3/6] drm/i915: use vmap in shmem_pin

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Message-ID: <20200922145323.GG32101@casper.infradead.org>
Date:   Tue, 22 Sep 2020 15:53:23 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>, x86@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 3/6] drm/i915: use vmap in shmem_pin_map

On Tue, Sep 22, 2020 at 04:39:06PM +0200, Christoph Hellwig wrote:
> On Tue, Sep 22, 2020 at 12:21:44PM +0100, Matthew Wilcox wrote:
> > Actually, vfree() will work today; I cc'd you on a documentation update
> > to make it clear that this is permitted.
> 
> vfree calls __free_pages, the i915 and a lot of other code calls
> put_page.  They are mostly the same, but not quite and everytime I
> look into that mess I'm more confused than before.
> 
> Can someone in the know write sensible documentation on when to use
> __free_page(s) vs put_page?

I started on that, and then I found a bug that's been lurking for 12
years, so that delayed the documentation somewhat.  The short answer is
that __free_pages() lets you free non-compound high-order pages while
put_page() can only free order-0 and compound pages.

I would really like to overhaul our memory allocation APIs:

current			new
__get_free_page(s)	alloc_page(s)
free_page(s)		free_page(s)
alloc_page(s)		get_free_page(s)
__free_pages		put_page_order

Then put_page() and put_page_order() are more obviously friends.

But I cannot imagine a world in which Linus says yes to that upheaval.
He's previous expressed dislike of the get_free_page() family of APIs,
and thinks all those callers should just use kmalloc().  Maybe we can
make that transition happen, now that kmalloc() aligns larger allocations.