Re: [PATCH 1/1] suspend: delete sys_sync()

From: Ming Lei
Date: Thu May 14 2015 - 20:40:14 EST

On Fri, May 15, 2015 at 8:34 AM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> On Friday, May 15, 2015 09:54:26 AM Dave Chinner wrote:
>> ng back On Thu, May 14, 2015 at 09:22:51AM +1000, NeilBrown wrote:
>> > On Mon, 11 May 2015 11:44:28 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>> >
>> > > On Fri, May 08, 2015 at 03:08:43AM -0400, Len Brown wrote:
>> > > > From: Len Brown <len.brown@xxxxxxxxx>
>> > > >
>> > > > Remove sys_sync() from the kernel's suspend flow.
>> > > >
>> > > > sys_sync() is extremely expensive in some configurations,
>> > > > and so the kernel should not force users to pay this cost
>> > > > on every suspend.
>> > >
>> > > Since when? Please explain what your use case is that makes this
>> > > so prohibitively expensive it needs to be removed.
>> > >
>> > > >
>> > > > The user-space utilities s2ram and s2disk choose to invoke sync() today.
>> > > > A user can invoke suspend directly via /sys/power/state to skip that cost.
>> > >
>> > > So, you want to have s2disk write all the dirty pages in memory to
>> > > the suspend image, rather than to the filesystem?
>> > >
>> > > Either way you have to write that dirty data to disk, but if you
>> > > write it to the suspend image, it then has to be loaded again on
>> > > resume, and then written again to the filesystem the system has
>> > > resumed. This doesn't seem very efficient to me....
>> > >
>> > > And, quite frankly, machines fail to resume from suspne dall the
>> > > time. e.g. run out of batteries when they are under s2ram
>> > > conditions, or s2disk fails because a kernel upgrade was done before
>> > > the s2disk and so can't be resumed. With your change, users lose all
>> > > the data that was buffered in memory before suspend, whereas right
>> > > now it is written to disk and so nothing is lost if the resume from
>> > > suspend fails for whatever reason.
>> > >
>> > > IOWs, I can see several good reasons why the sys_sync() needs to
>> > > remain in the suspend code. User data safety and filesystem
>> > > integrity is far, far more important than a couple of seconds
>> > > improvement in suspend speed....
>> >
>> > To be honest, this sounds like superstition and fear, not science and fact.
>> >
>> > "filesystem integrity" is not an issue for the fast majority of filesystems
>> > which use journalling to ensure continued integrity even after a crash. I
>> > think even XFS does that :-)
>> It has nothing to do with journalling, and everything to do with
>> bring filesystems to an *idle state* before suspend runs. We have a
>> long history of bug reports with XFS that go: suspend, resume, XFS
>> almost immediately detects corruption, shuts down.
>> The problem is that "sync" doesn't make the filesystem idle - XFs
>> has *lots* of background work going on, and if we aren't *real
>> careful* the filesystem is still doing work while the hardware gets
>> powerd down and the suspend image is being taken. the result is on
>> resume that the on-disk filesystem state does not match the memory
>> image pulled back from resume, and we get shutdowns.
>> sys_sync() does not guarantee a filesystem is idle - it guarantees
>> the data in memory is recoverable, butit doesn't stop the filesystem
>> from doing things like writing back metadata or running background
>> cleaup tasks. If those aren't stopped properly, then we get into
>> the state where in-memory and on-disk state get out of whack. And
>> s2ram can have these problems too, because if there is IO in flight
>> when the hardware is powered down, that IO is lost....
>> Every time some piece of generic infrastructure changes behaviour
>> w.r.t. suspend/resume, we get a new set of problems being reported
>> by users. It's extremely hard to test for these problems and it
>> might take months of occasional corruption reports from a user to
>> isolate it to being a suspend/resume problem. It's a game of
>> whack-a-mole, because quite often they come down to the fact that
>> something changed and nobody in the XFS world knew they had to now
>> set an different initialisation flag on some structure or workqueue
>> to make it work the way it needed to work.
>> Go back an look at the history of sys_sync() in suspend discussions
>> over the past 10 years. You'll find me saying exactly the same
>> thing again and again about sys_sync(): it does not guarantee the
>> filesystem is in an idle or coherent, unchanging state, and nothing
>> in the suspend code tells the filesystem to enter an idle or frozen
>> state. We actually have mechanisms for doing this - we use it in the
>> storage layers to idle the filesystem while we do things like *take
>> a snapshot*.
>> What is the mechanism suspend to disk uses? It *takes a snapshot* of
>> system state, written to disk. It's supposed to be consistent, and
>> the only way you can guarantee the state of an active, mounted
>> filesystem has consistent in-memory state and on-disk state and
>> that it won't get changed is to *freeze the filesystem*.
>> Removing the sync is only going to make this problem worse because
>> the delta between on-disk and in-memory state is going to be much,
>> much larger. There is also likely to be significant filesystem
>> activity occurring when the filesystem has all it's background
>> threads and work queues abruptly frozen with no warning or
>> co-ordination, which makes it impossible for anyone to test
>> suspend/resume reliably.
>> Sorry for the long rant, but I've been saying the same thing for 10
>> years, which is abotu as long as I've been dealing with filesystem
>> corruptions that have resulted from suspend/resume.
> Well, the change proposed by Len is *only* about suspend-to-RAM and
> similar. It is *not* about suspend-to-disk, so pretty please let's
> not confuse things.
> So what problems may arise specifically in the suspend-to-RAM case if
> we remove the unconditional sys_sync() from its code path?

Data loss may be caused for hotplug storage(like USB), or all storage
when power is exhausted during suspend.

Is there obvious advantage to remove sys_sync() in the case?

Ming Lei
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