Re: [fuse-devel] [fuse] What happens with dirty pages on NOTIFY_INVAL_INODE?

[Miklos Szeredi <miklos@szeredi.hu>]

On Mon, Sep 24, 2018 at 10:43 AM, Nikolaus Rath <Nikolaus@rath.org> wrote:
> On Sep 24 2018, Miklos Szeredi <miklos@szeredi.hu> wrote:
>> On Mon, Sep 24, 2018 at 10:06 AM, Nikolaus Rath <Nikolaus@rath.org> wrote:
>>> Hi,
>>>
>>> What happens with dirty pages when a (writeback-cache enabled) FUSE
>>> filesystem sends a NOTIFY_INVAL_INODE request? Are they dropped?
>>> flushed?
>>
>> Haven't tried, but AFAICS it flushes dirty pages and waits on
>> writeback for these.
>
> "waits on writeback" means "wait until the write requests have
> completed"?

Appears to be so.  An so there we have the blocking notification
you've asked about.

>
>> However, it doesn't wait on already queued
>> writes.  So it's a bit of a mess at the moment.
>>
>>>
>>> To me neither behaviour seems correct...
>>
>> What would be the correct operation be if neither flushing not
>> dropping them is correct?
>
> What about returning an error?
>
> My thinking is that if the filesystem issues an inval request, then the
> data has already been changed/disappeared. So a writeback at this point
> would most likely not do the right thing - since it would partially
> write back old data that has actually been mutated.
>
> Similarly, just dropping the cache seems bad because most likely this
> causes data loss for the new data that hasn't been flushed.

This is a pretty complicated issue.  For reference we should look at
what NFS is doing, because NFS has a long history and likely that
behavior is the most acceptable when cached writes are mixed with
concurrent remote updates.

And I don't think NFS will report an error in such a situation.   More
likely it will accept the reordered writes.

Thanks,
Miklos