On Tue, Nov 16, 2021 at 9:54 AM Rafael J. Wysocki [off-list ref] wrote:

On Mon, Nov 15, 2021 at 6:13 PM Evan Green [off-list ref] wrote:

quoted

Gentle bump.


On Fri, Oct 29, 2021 at 12:24 PM Evan Green [off-list ref] wrote:

quoted

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 <redacted>
---

 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