linux-kernel - Re: [PATCH v2 1/1] fs/splice: add missing callback for inaccessible pages

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Date:   Thu, 30 Apr 2020 15:20:07 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        viro@...iv.linux.org.uk
Cc:     david@...hat.com, akpm@...ux-foundation.org, aarcange@...hat.com,
        linux-mm@...ck.org, frankja@...ux.ibm.com, sfr@...b.auug.org.au,
        jhubbard@...dia.com, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, jack@...e.cz, kirill@...temov.name,
        peterz@...radead.org, sean.j.christopherson@...el.com,
        Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH v2 1/1] fs/splice: add missing callback for inaccessible
 pages

One other thing...  The gup code will not take references on ref-frozen
pages:

> static inline __must_check bool try_get_page(struct page *page)
> {
>         page = compound_head(page);
>         if (WARN_ON_ONCE(page_ref_count(page) <= 0))
>                 return false;
>         page_ref_inc(page);
>         return true;
> }

*But*, notice that the path that skips taking a ref is also a
WARN_ON_ONCE().  Basically, if you get to try_get_page() on a ref-frozen
page, it's considered buggy.  This makes sense because you fundamentally
can't freeze refs on a page that might have more refs taken on it.

I think all the other users do this by ensuring that any PTE that could
be gup'd is set non-present before the refs are frozen and remote TLBs
are flushed which also ensures no GUPs are running.  I don't know if the
s390 code has some other way of preventing GUPs, but leaving Present=1
PTEs while you freeze refs would be quite troublesome on x86.