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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 9 Jan 2021 17:19:51 -0800
Message-ID: <CAHk-=wj5=1DKbQut1-21EwQbMSghNL3KOSd82rNrBhuG9+eekA@mail.gmail.com>
Subject: Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Yu Zhao <yuzhao@google.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Jason Gunthorpe <jgg@ziepe.ca>, Jan Kara <jack@suse.cz>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Nadav Amit <nadav.amit@gmail.com>, Jens Axboe <axboe@kernel.dk>
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On Sat, Jan 9, 2021 at 4:55 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> What part of "clear_refs is the _least_ important of the three cases"
> are you not willing to understand?

In fact, I couldn't even turn on that code with my normal config,
because it depends on CONFIG_CHECKPOINT_RESTORE that I didn't even
have enabled.

IOW, that code is some special-case stuff, and instead of messing up
the rest of the VM, it should be made to conform to all the normal VM
rules and requirements.

Here's two patches to basically start doing that.

The first one is the same one I already sent out earlier, fixing the
locking. And yes, it can be improved upon, but before improving on it,
let's _fix_ the code.

The second is a trivial "oh, look, I can see that the page is pinned,
soft-dirty cannot work so don't do it then". Again, it can be improved
upon, most particularly by doing the same (simple) tests for the
hugepage case too, which I didn't do.

Note: I have not a single actual user of this code that I can test
with, so this is all ENTIRELY untested.

IOW, I am in no way claiming that these patches are perfect and
correct, and the only way to do things.

But what I _am_ claiming is that this clear_refs code (and the UFFD
code) is of secondary importance, and instead of messing up the core
VM, we should fix these special cases to not do bad things.

It really is that simple.

And no, I didn't make the UFFDIO_WRITEPROTECT code take the mmap_sem
for writing. For whoever wants to look at that, it's
mwriteprotect_range() in mm/userfaultfd.c and the fix is literally to
turn the read-lock (and unlock) into a write-lock (and unlock).

           Linus

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