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Subject: Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for
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To:     Jann Horn <jannh@google.com>, Peter Xu <peterx@redhat.com>
CC:     Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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References: <20200921211744.24758-1-peterx@redhat.com>
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From:   John Hubbard <jhubbard@nvidia.com>
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On 9/21/20 2:55 PM, Jann Horn wrote:
> On Mon, Sep 21, 2020 at 11:20 PM Peter Xu <peterx@redhat.com> wrote:
...
> I dislike the whole pin_user_pages() concept because (as far as I
> understand) it fundamentally tries to fix a problem in the subset of
> cases that are more likely to occur in practice (long-term pins
> overlapping with things like writeback), and ignores the rarer cases
> ("short-term" GUP).
> 

Well, no, that's not really fair. pin_user_pages() provides a key
prerequisite to fixing *all* of the bugs in that area, not just a
subset. The 5 cases in Documentation/core-api/pin_user_pages.rst cover
this pretty well. Or if they don't, let me know and I'll have another
pass at it.

The case for a "pin count" that is (logically) separate from a
page->_refcount is real, and it fixes real problems. An elevated
refcount can be caused by a lot of things, but it can normally be waited
for and/or retried. The FOLL_PIN pages cannot.

Of course, a valid remaining criticism of the situation is, "why not
just *always* mark any of these pages as "dma-pinned"? In other words,
why even have a separate gup/pup API? And in fact, perhaps eventually
we'll just get rid of the get_user_pages*() side of it. But the pin
count will need to remain, in order to discern between DMA pins and
temporary refcount boosts.

thanks,
-- 
John Hubbard
NVIDIA