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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 22 Jun 2021 11:57:10 -0700
Message-ID: <CAHk-=whDyrOMJMZBxtTUaMeuSd_dafiR_DFCwsLSbJuLwEuYsw@mail.gmail.com>
Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user
 buffer pages"?
To:     Nadav Amit <nadav.amit@gmail.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        David Howells <dhowells@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>, "Ted Ts'o" <tytso@mit.edu>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Jun 22, 2021 at 11:51 AM Nadav Amit <nadav.amit@gmail.com> wrote:
> Just reminding the alternative (in the RFC that I mentioned before):
> a vDSO exception table entry for a memory accessing function in the
> vDSO. It then behaves as a sort of MADV_WILLNEED for the faulting
> page if an exception is triggered. Unlike MADV_WILLNEED it maps the
> page if no IO is needed. It can return through a register whether
> the page was present or not.

Yeah, that looks like a user-space equivalent.

And thanks to the vdso, it doesn't need to support all architectures.
Unlike a kernel model would (but yes, a kernel model could then have a
fallback for the non-prefetching synchronous case instead, so I guess
we could just do one architecture at a time).

               Linus