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    SubjectRe: [PATCH v2 23/26] userfaultfd: wp: don't wake up when doing write protect
    On Mon, Feb 25, 2019 at 04:58:46PM +0800, Peter Xu wrote:
    > On Thu, Feb 21, 2019 at 01:36:54PM -0500, Jerome Glisse wrote:
    > > On Tue, Feb 12, 2019 at 10:56:29AM +0800, Peter Xu wrote:
    > > > It does not make sense to try to wake up any waiting thread when we're
    > > > write-protecting a memory region. Only wake up when resolving a write
    > > > protected page fault.
    > > >
    > > > Signed-off-by: Peter Xu <peterx@redhat.com>
    > >
    > > I am bit confuse here, see below.
    > >
    > > > ---
    > > > fs/userfaultfd.c | 13 ++++++++-----
    > > > 1 file changed, 8 insertions(+), 5 deletions(-)
    > > >
    > > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
    > > > index 81962d62520c..f1f61a0278c2 100644
    > > > --- a/fs/userfaultfd.c
    > > > +++ b/fs/userfaultfd.c
    > > > @@ -1771,6 +1771,7 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
    > > > struct uffdio_writeprotect uffdio_wp;
    > > > struct uffdio_writeprotect __user *user_uffdio_wp;
    > > > struct userfaultfd_wake_range range;
    > > > + bool mode_wp, mode_dontwake;
    > > >
    > > > if (READ_ONCE(ctx->mmap_changing))
    > > > return -EAGAIN;
    > > > @@ -1789,18 +1790,20 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
    > > > if (uffdio_wp.mode & ~(UFFDIO_WRITEPROTECT_MODE_DONTWAKE |
    > > > UFFDIO_WRITEPROTECT_MODE_WP))
    > > > return -EINVAL;
    > > > - if ((uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP) &&
    > > > - (uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE))
    >
    > [1]
    >
    > > > +
    > > > + mode_wp = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP;
    > > > + mode_dontwake = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE;
    > > > +
    > > > + if (mode_wp && mode_dontwake)
    >
    > [2]
    >
    > > > return -EINVAL;
    > >
    > > I am confuse by the logic here. DONTWAKE means do not wake any waiting
    > > thread right ? So if the patch header it seems to me the logic should
    > > be:
    > > if (mode_wp && !mode_dontwake)
    > > return -EINVAL;
    >
    > This should be the most common case when we want to write protect a
    > page (or a set of pages). I'll explain more details below...
    >
    > >
    > > At very least this part does seems to mean the opposite of what the
    > > commit message says.
    >
    > Let me paste the matrix to be clear on these flags:
    >
    > |------+-------------------------+------------------------------|
    > | | dontwake=0 | dontwake=1 |
    > |------+-------------------------+------------------------------|
    > | wp=0 | (a) resolve pf, do wake | (b) resolve pf only, no wake |
    > | wp=1 | (c) wp page range | (d) invalid |
    > |------+-------------------------+------------------------------|
    >
    > Above check at [1] was checking against case (d) in the matrix. It is
    > indeed an invalid condition because when we want to write protect a
    > page we should not try to wake up any thread, so the donewake
    > parameter is actually useless (we'll always do that). And above [2]
    > is simply rewritting [1] with the new variables.

    I think (c) is "wp range and wake the thread", and (d) is "wp and DONT
    wake".


    > >
    > > >
    > > > ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
    > > > - uffdio_wp.range.len, uffdio_wp.mode &
    > > > - UFFDIO_WRITEPROTECT_MODE_WP,
    > > > + uffdio_wp.range.len, mode_wp,
    > > > &ctx->mmap_changing);
    > > > if (ret)
    > > > return ret;
    > > >
    > > > - if (!(uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE)) {
    > > > + if (!mode_wp && !mode_dontwake) {
    > >
    > > This part match the commit message :)
    >
    > Here is what the patch really want to change: before this patch we'll
    > even call wake_userfault() below for case (c) while it doesn't really
    > make too much sense IMHO. After this patch we'll only do the wakeup
    > for (a,b).

    Waking up the thread after the last region is write-protected would make
    sense. Not much savings for lots of ranges, though.

    > >
    > > > range.start = uffdio_wp.range.start;
    > > > range.len = uffdio_wp.range.len;
    > > > wake_userfault(ctx, &range);
    >
    > Thanks,
    >
    > --
    > Peter Xu
    >

    --
    Sincerely yours,
    Mike.

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