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From:   Evan Green <evgreen@chromium.org>
Date:   Tue, 16 Nov 2021 12:22:11 -0800
Message-ID: <CAE=gft6CjUhkcrmcjVEOp5S+rgqN1_ZGTKbK0DierTanu0d16A@mail.gmail.com>
Subject: Re: [PATCH] PM / hibernate: Fix snapshot partial write lengths
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Pavel Machek <pavel@ucw.cz>, Len Brown <len.brown@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>
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On Tue, Nov 16, 2021 at 9:54 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Mon, Nov 15, 2021 at 6:13 PM Evan Green <evgreen@chromium.org> wrote:
> >
> > Gentle bump.
> >
> >
> > On Fri, Oct 29, 2021 at 12:24 PM Evan Green <evgreen@chromium.org> wrote:
> > >
> > > snapshot_write() is inappropriately limiting the amount of data that can
> > > be written in cases where a partial page has already been written. For
> > > example, one would expect to be able to write 1 byte, then 4095 bytes to
> > > the snapshot device, and have both of those complete fully (since now
> > > we're aligned to a page again). But what ends up happening is we write 1
> > > byte, then 4094/4095 bytes complete successfully.
> > >
> > > The reason is that simple_write_to_buffer()'s second argument is the
> > > total size of the buffer, not the size of the buffer minus the offset.
> > > Since simple_write_to_buffer() accounts for the offset in its
> > > implementation, snapshot_write() can just pass the full page size
> > > directly down.
> > >
> > > Signed-off-by: Evan Green <evgreen@chromium.org>
> > > ---
> > >
> > >  kernel/power/user.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/power/user.c b/kernel/power/user.c
> > > index 740723bb388524..ad241b4ff64c58 100644
> > > --- a/kernel/power/user.c
> > > +++ b/kernel/power/user.c
> > > @@ -177,7 +177,7 @@ static ssize_t snapshot_write(struct file *filp, const char __user *buf,
> > >                 if (res <= 0)
> > >                         goto unlock;
> > >         } else {
> > > -               res = PAGE_SIZE - pg_offp;
> > > +               res = PAGE_SIZE;
> > >         }
> > >
> > >         if (!data_of(data->handle)) {
> > > --
>
> Do you actually see this problem in practice?

Yes. I may fire up another thread to explain why I'm stuck doing a
partial page write, and how I might be able to stop doing that in the
future with some kernel help. But either way, this is a bug.
-Evan