Re: [PATCH] PM / hibernate: Fix snapshot partial write lengths

[Evan Green]

On Tue, Nov 30, 2021 at 3:46 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>
> On Mon, Nov 29, 2021 at 5:50 PM Evan Green <evgreen@xxxxxxxxxxxx> wrote:
> >
> > On Wed, Nov 24, 2021 at 4:54 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Nov 16, 2021 at 9:22 PM Evan Green <evgreen@xxxxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Nov 16, 2021 at 9:54 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Nov 15, 2021 at 6:13 PM Evan Green <evgreen@xxxxxxxxxxxx> wrote:
> > > > > >
> > > > > > Gentle bump.
> > > > > >
> > > > > >
> > > > > > On Fri, Oct 29, 2021 at 12:24 PM Evan Green <evgreen@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > > > > > ---
> > > > > > >
> > > > > > >  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.
> > >
> > > OK, patch applied as 5.16-rc material.
> > >
> > > I guess it should go into -stable kernels too?
> >
> > Yes, putting it into -stable would make sense also. I should have CCed
> > them originally, doing that now.
>
> Well, you need to point them to the upstream commit to backport.
>
> In this particular case it would be
>
> commit 88a5045f176b78c33a269a30a7b146e99c550bd9 (pm-sleep)
> Author: Evan Green <evgreen@xxxxxxxxxxxx>
> Date:   Fri Oct 29 12:24:22 2021 -0700
>
>    PM: hibernate: Fix snapshot partial write lengths
>
> I'll send an inclusion request for this.  I guess it should go into
> all of the applicable -stable series, right?

Correct. From what I can tell this code and simple_write_to_buffer()
haven't changed since its introduction in 2010, so my patch should
have had a fixes tag like:

Fixes: d3c1b24c50e8 ("PM / Hibernate: Snapshot cleanup")

Thanks for doing that Rafael, sorry for flubbing the stable aspect.

-Evan