Date: Tue, 23 Jun 2015 23:41:41 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, qemu-devel@nongnu.org, kvm@vger.kernel.org,
        linux-api@vger.kernel.org, Pavel Emelyanov <xemul@parallels.com>,
        Sanidhya Kashyap <sanidhya.gatech@gmail.com>,
        zhang.zhanghailiang@huawei.com,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andres Lagar-Cavilla <andreslc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>, Rik van Riel <riel@redhat.com>,
        Mel Gorman <mgorman@suse.de>, Andy Lutomirski <luto@amacapital.net>,
        Hugh Dickins <hughd@google.com>, Peter Feiner <pfeiner@google.com>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Huangpeng (Peter)" <peter.huangpeng@huawei.com>
Subject: Re: [PATCH 10/23] userfaultfd: add new syscall to provide memory
 externalization
Message-ID: <20150623214141.GB4312@redhat.com>
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 <1431624680-20153-11-git-send-email-aarcange@redhat.com>
 <5589ACC3.3060401@intel.com>
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Hi Dave,

On Tue, Jun 23, 2015 at 12:00:19PM -0700, Dave Hansen wrote:
> Down in userfaultfd_wake_function(), it looks like you intended for a
> len=0 to mean "wake all".  But the validate_range() that we do from
> userspace has a !len check in it, which keeps us from passing a len=0 in
> from userspace.
> Was that "wake all" for some internal use, or is the check too strict?

It's for internal use or userfaultfd_release that has to wake them all
(after setting ctx->released) if the uffd is closed. It avoids to
enlarge the structure by depending on the invariant that userland
cannot pass len=0.

If we'd accept len=0 from userland as valid, I'd be safer if it does
nothing like in madvise, I doubt we want to expose this non standard
kernel internal behavior to userland.

> I was trying to use the wake ioctl after an madvise() (as opposed to
> filling things in using a userfd copy).

madvise will return 0 if len=0, mremap would return -EINVAL if new_len
is zero, mmap also returns -EINVAL if len is 0, not all MM syscalls
are as permissive as madvise. Can't you pass the same len you pass to
madvise to UFFDIO_WAKE (or just skip the call if the madvise len is
zero)?

Thanks,
Andrea
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