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Date: Mon, 30 Sep 2013 17:38:33 +0200 From: Miklos Szeredi <miklos@...redi.hu> To: Ric Wheeler <rwheeler@...hat.com> Cc: "J. Bruce Fields" <bfields@...ldses.org>, "Myklebust, Trond" <Trond.Myklebust@...app.com>, Zach Brown <zab@...hat.com>, Anna Schumaker <schumaker.anna@...il.com>, Kernel Mailing List <linux-kernel@...r.kernel.org>, Linux-Fsdevel <linux-fsdevel@...r.kernel.org>, "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>, "Schumaker, Bryan" <Bryan.Schumaker@...app.com>, "Martin K. Petersen" <mkp@....net>, Jens Axboe <axboe@...nel.dk>, Mark Fasheh <mfasheh@...e.com>, Joel Becker <jlbec@...lplan.org>, Eric Wong <normalperson@...t.net> Subject: Re: [RFC] extending splice for copy offloading On Mon, Sep 30, 2013 at 4:28 PM, Ric Wheeler <rwheeler@...hat.com> wrote: > On 09/30/2013 10:24 AM, Miklos Szeredi wrote: >> >> On Mon, Sep 30, 2013 at 4:52 PM, Ric Wheeler <rwheeler@...hat.com> wrote: >>> >>> On 09/30/2013 10:51 AM, Miklos Szeredi wrote: >>>> >>>> On Mon, Sep 30, 2013 at 4:34 PM, J. Bruce Fields <bfields@...ldses.org> >>>> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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