Re: [RFC] extending splice for copy offloading

[Ric Wheeler]

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?
>
> Thanks,
> Miklos

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.

I don't see that (2) is interesting or really needed to be done in the kernel. 
If nothing else, it tends to confuse the discussion....

ric

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