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

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Date:   Tue, 16 Nov 2021 18:54:22 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Evan Green <evgreen@...omium.org>
Cc:     Pavel Machek <pavel@....cz>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] PM / hibernate: Fix snapshot partial write lengths

On Mon, Nov 15, 2021 at 6:13 PM Evan Green <evgreen@...omium.org> wrote:
>
> Gentle bump.
>
>
> On Fri, Oct 29, 2021 at 12:24 PM Evan Green <evgreen@...omium.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@...omium.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?