Re: [RFC] extending splice for copy offloading

[Ric Wheeler]

On 09/30/2013 10:38 AM, Miklos Szeredi wrote:
> On Mon, Sep 30, 2013 at 4:28 PM, Ric Wheeler <rwheeler@xxxxxxxxxx> wrote:
> > On 09/30/2013 10:24 AM, Miklos Szeredi wrote:
> > > On Mon, Sep 30, 2013 at 4:52 PM, Ric Wheeler <rwheeler@xxxxxxxxxx> wrote:
> > > > On 09/30/2013 10:51 AM, Miklos Szeredi wrote:
> > > > > On Mon, Sep 30, 2013 at 4:34 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx>
> > > > > wrote:
> > > > > > > My other worry is about interruptibility/restartability.  Ideas?
> > > > > > >
> > > > > > > What happens on splice(from, to, 4G) and it's a non-reflink copy?
> > > > > > > Can the page cache copy be made restartable?   Or should splice() be
> > > > > > > allowed to return a short count?  What happens on (non-reflink) remote
> > > > > > > copies and huge request sizes?
> > > > > > If I were writing an application that required copies to be
> > > > > > restartable,
> > > > > > I'd probably use the largest possible range in the reflink case but
> > > > > > break the copy into smaller chunks in the splice case.
> > > > > The app really doesn't want to care about that.  And it doesn't want
> > > > > to care about restartability, etc..  It's something the *kernel* has
> > > > > to care about.   You just can't have uninterruptible syscalls that
> > > > > sleep for a "long" time, otherwise first you'll just have annoyed
> > > > > users pressing ^C in vain; then, if the sleep is even longer, warnings
> > > > > about task sleeping too long.
> > > > >
> > > > > One idea is letting splice() return a short count, and so the app can
> > > > > safely issue SIZE_MAX requests and the kernel can decide if it can
> > > > > copy the whole file in one go or if it wants to do it in smaller
> > > > > chunks.
> > > > You cannot rely on a short count. That implies that an offloaded copy
> > > > starts
> > > > at byte 0 and the short count first bytes are all valid.
> > > Huh?
> > >
> > > - app calls splice(from, 0, to, 0, SIZE_MAX)
> > >    1) VFS calls ->direct_splice(from, 0,  to, 0, SIZE_MAX)
> > >       1.a) fs reflinks the whole file in a jiffy and returns the size of
> > > the file
> > >       1 b) fs does copy offload of, say, 64MB and returns 64M
> > >    2) VFS does page copy of, say, 1MB and returns 1MB
> > > - app calls splice(from, X, to, X, SIZE_MAX) where X is the new offset
> > > ...
> > >
> > > The point is: the app is always doing the same (incrementing offset
> > > with the return value from splice) and the kernel can decide what is
> > > the best size it can service within a single uninterruptible syscall.
> > >
> > > Wouldn't that work?
> > No.
> >
> > Keep in mind that the offload operation in (1) might fail partially. The
> > target file (the copy) is allocated, the question is what ranges have valid
> > data.
> You are talking about case 1.a, right?  So if the offload copy 0-64MB
> fails partially, we return failure from splice, yet some of the copy
> did succeed.  Is that the problem?  Why?
>
> Thanks,
> Miklos

The way the array based offload (and some software side reflink works) is not a 
byte by byte copy. We cannot assume that a valid count can be returned or that 
such a count would be an indication of a sequential segment of good data.  The 
whole thing would normally have to be reissued.

To make that a true assumption, you would have to mandate that in each of the 
specifications (and sw targets)...

ric

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/