On Mon, Feb 26, 2018 at 1:44 PM, Jan Kara <jack@xxxxxxx> wrote:
> On Mon 26-02-18 10:25:55, Rafael J. Wysocki wrote:
>> On Sun, Feb 25, 2018 at 6:22 PM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx> wrote:
>> > Ah, yes, I see your point now. So for filesystems we really don't
>> > care if its suspend or hibernation, we just need to freeze in the right
>> > order. So long as we get that order right we should be OK.
>>
>> Generally, yes.
>>
>> System suspend/resume (S2R, S2I), however, doesn't really require the
>> flushing part, so in principle the full-blown fs freezing is not
>> necessary in that case, strictly speaking. The ordering of writes
>> still has to be preserved in this case (that is, writes to persistent
>> storage in the presence of a suspend-resume cycle must occur in the
>> same order as without the suspend-resume), but it doesn't matter too
>> much when the writes actually happen.
>
> I agree that in principle we don't have to flush all dirty data from page
> cache before system suspend (I believe this is what you are speaking about
> here, isn't it?).
Yes, it is.
> However from implementation POV it is much simpler that
> way as otherwise processes get blocked in the kernel in unexpected places
> waiting for locks held by blocked flusher threads etc.
Understood.
>> They may be carried out before
>> the suspend or after the resume or somewhere during one of them: all
>> should be fine so long as the ordering of writes doesn't change as a
>> result of the suspend-resume (and suspend-resume failures are like
>> surprise resets from the fs perspective in that case, so journaling
>> should be sufficient to recover from them).
>
> Err, I don't follow how suspend failure looks like surprise reset to the
> fs. If suspend fails, we just thaw the filesystem and off we go, no IO
> lost.
Right.
> If you speak about a situation where you suspend but then boot
> without resuming - yes, that looks like a surprise reset and it behaves
> that way already today.
I was talking about the latter.
>> Of course, the full-blown freezing will work for system suspend too,
>> but it may be high-latency which is not desirable in some scenarios
>> utilizing system suspend/resume ("dark resume" or "lucid sleep"
>> scenarios, for example). That will be a concern in the long run (it
>> kind of is a concern already today), so I would consider
>> special-casing it from the outset.
>
> Understood but that would require considerable amount of work on the fs
> side and the problem is hard enough as is :)
Fair enough. :-)
> And switching the freeze implementation to avoid sync(2) if asked to is quite
> independent from implementing system suspend to use fs freezing.
Agreed.
Thanks,
Rafael
