From: Kanchan Joshi <joshiiitr@gmail.com> To: Damien Le Moal <Damien.LeMoal@wdc.com> Cc: Jens Axboe <axboe@kernel.dk>, Pavel Begunkov <asml.silence@gmail.com>, Kanchan Joshi <joshi.k@samsung.com>, "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>, "bcrl@kvack.org" <bcrl@kvack.org>, Matthew Wilcox <willy@infradead.org>, "hch@infradead.org" <hch@infradead.org>, "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-aio@kvack.org" <linux-aio@kvack.org>, "io-uring@vger.kernel.org" <io-uring@vger.kernel.org>, "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>, "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>, SelvaKumar S <selvakuma.s1@samsung.com>, Nitesh Shetty <nj.shetty@samsung.com>, Javier Gonzalez <javier.gonz@samsung.com> Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append Date: Fri, 31 Jul 2020 12:38:27 +0530 Message-ID: <CA+1E3rJ5j6MeG3O5Xa7unWLMRz6BacvLVN8xpeEz6AVyJWT55Q@mail.gmail.com> (raw) In-Reply-To: <MWHPR04MB3758DC08EA17780E498E9EC0E74E0@MWHPR04MB3758.namprd04.prod.outlook.com> On Fri, Jul 31, 2020 at 12:12 PM Damien Le Moal <Damien.LeMoal@wdc.com> wrote: > > On 2020/07/31 3:26, Kanchan Joshi wrote: > > On Thu, Jul 30, 2020 at 11:24 PM Jens Axboe <axboe@kernel.dk> wrote: > >> > >> On 7/30/20 11:51 AM, Kanchan Joshi wrote: > >>> On Thu, Jul 30, 2020 at 11:10 PM Pavel Begunkov <asml.silence@gmail.com> wrote: > >>>> > >>>> On 30/07/2020 20:16, Jens Axboe wrote: > >>>>> On 7/30/20 10:26 AM, Pavel Begunkov wrote: > >>>>>> On 30/07/2020 19:13, Jens Axboe wrote: > >>>>>>> On 7/30/20 10:08 AM, Pavel Begunkov wrote: > >>>>>>>> On 27/07/2020 23:34, Jens Axboe wrote: > >>>>>>>>> On 7/27/20 1:16 PM, Kanchan Joshi wrote: > >>>>>>>>>> On Fri, Jul 24, 2020 at 10:00 PM Jens Axboe <axboe@kernel.dk> wrote: > >>>>>>>>>>> > >>>>>>>>>>> On 7/24/20 9:49 AM, Kanchan Joshi wrote: > >>>>>>>>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c > >>>>>>>>>>>> index 7809ab2..6510cf5 100644 > >>>>>>>>>>>> --- a/fs/io_uring.c > >>>>>>>>>>>> +++ b/fs/io_uring.c > >>>>>>>>>>>> @@ -1284,8 +1301,15 @@ static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags) > >>>>>>>>>>>> cqe = io_get_cqring(ctx); > >>>>>>>>>>>> if (likely(cqe)) { > >>>>>>>>>>>> WRITE_ONCE(cqe->user_data, req->user_data); > >>>>>>>>>>>> - WRITE_ONCE(cqe->res, res); > >>>>>>>>>>>> - WRITE_ONCE(cqe->flags, cflags); > >>>>>>>>>>>> + if (unlikely(req->flags & REQ_F_ZONE_APPEND)) { > >>>>>>>>>>>> + if (likely(res > 0)) > >>>>>>>>>>>> + WRITE_ONCE(cqe->res64, req->rw.append_offset); > >>>>>>>>>>>> + else > >>>>>>>>>>>> + WRITE_ONCE(cqe->res64, res); > >>>>>>>>>>>> + } else { > >>>>>>>>>>>> + WRITE_ONCE(cqe->res, res); > >>>>>>>>>>>> + WRITE_ONCE(cqe->flags, cflags); > >>>>>>>>>>>> + } > >>>>>>>>>>> > >>>>>>>>>>> This would be nice to keep out of the fast path, if possible. > >>>>>>>>>> > >>>>>>>>>> I was thinking of keeping a function-pointer (in io_kiocb) during > >>>>>>>>>> submission. That would have avoided this check......but argument count > >>>>>>>>>> differs, so it did not add up. > >>>>>>>>> > >>>>>>>>> But that'd grow the io_kiocb just for this use case, which is arguably > >>>>>>>>> even worse. Unless you can keep it in the per-request private data, > >>>>>>>>> but there's no more room there for the regular read/write side. > >>>>>>>>> > >>>>>>>>>>>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h > >>>>>>>>>>>> index 92c2269..2580d93 100644 > >>>>>>>>>>>> --- a/include/uapi/linux/io_uring.h > >>>>>>>>>>>> +++ b/include/uapi/linux/io_uring.h > >>>>>>>>>>>> @@ -156,8 +156,13 @@ enum { > >>>>>>>>>>>> */ > >>>>>>>>>>>> struct io_uring_cqe { > >>>>>>>>>>>> __u64 user_data; /* sqe->data submission passed back */ > >>>>>>>>>>>> - __s32 res; /* result code for this event */ > >>>>>>>>>>>> - __u32 flags; > >>>>>>>>>>>> + union { > >>>>>>>>>>>> + struct { > >>>>>>>>>>>> + __s32 res; /* result code for this event */ > >>>>>>>>>>>> + __u32 flags; > >>>>>>>>>>>> + }; > >>>>>>>>>>>> + __s64 res64; /* appending offset for zone append */ > >>>>>>>>>>>> + }; > >>>>>>>>>>>> }; > >>>>>>>>>>> > >>>>>>>>>>> Is this a compatible change, both for now but also going forward? You > >>>>>>>>>>> could randomly have IORING_CQE_F_BUFFER set, or any other future flags. > >>>>>>>>>> > >>>>>>>>>> Sorry, I didn't quite understand the concern. CQE_F_BUFFER is not > >>>>>>>>>> used/set for write currently, so it looked compatible at this point. > >>>>>>>>> > >>>>>>>>> Not worried about that, since we won't ever use that for writes. But it > >>>>>>>>> is a potential headache down the line for other flags, if they apply to > >>>>>>>>> normal writes. > >>>>>>>>> > >>>>>>>>>> Yes, no room for future flags for this operation. > >>>>>>>>>> Do you see any other way to enable this support in io-uring? > >>>>>>>>> > >>>>>>>>> Honestly I think the only viable option is as we discussed previously, > >>>>>>>>> pass in a pointer to a 64-bit type where we can copy the additional > >>>>>>>>> completion information to. > >>>>>>>> > >>>>>>>> TBH, I hate the idea of such overhead/latency at times when SSDs can > >>>>>>>> serve writes in less than 10ms. Any chance you measured how long does it > >>>>>>> > >>>>>>> 10us? :-) > >>>>>> > >>>>>> Hah, 10us indeed :) > >>>>>> > >>>>>>> > >>>>>>>> take to drag through task_work? > >>>>>>> > >>>>>>> A 64-bit value copy is really not a lot of overhead... But yes, we'd > >>>>>>> need to push the completion through task_work at that point, as we can't > >>>>>>> do it from the completion side. That's not a lot of overhead, and most > >>>>>>> notably, it's overhead that only affects this particular type. > >>>>>>> > >>>>>>> That's not a bad starting point, and something that can always be > >>>>>>> optimized later if need be. But I seriously doubt it'd be anything to > >>>>>>> worry about. > >>>>>> > >>>>>> I probably need to look myself how it's really scheduled, but if you don't > >>>>>> mind, here is a quick question: if we do work_add(task) when the task is > >>>>>> running in the userspace, wouldn't the work execution wait until the next > >>>>>> syscall/allotted time ends up? > >>>>> > >>>>> It'll get the task to enter the kernel, just like signal delivery. The only > >>>>> tricky part is really if we have a dependency waiting in the kernel, like > >>>>> the recent eventfd fix. > >>>> > >>>> I see, thanks for sorting this out! > >>> > >>> Few more doubts about this (please mark me wrong if that is the case): > >>> > >>> - Task-work makes me feel like N completions waiting to be served by > >>> single task. > >>> Currently completions keep arriving and CQEs would be updated with > >>> result, but the user-space (submitter task) would not be poked. > >>> > >>> - Completion-code will set the task-work. But post that it cannot go > >>> immediately to its regular business of picking cqe and updating > >>> res/flags, as we cannot afford user-space to see the cqe before the > >>> pointer update. So it seems completion-code needs to spawn another > >>> work which will allocate/update cqe after waiting for pointer-update > >>> from task-work? > >> > >> The task work would post the completion CQE for the request after > >> writing the offset. > > > > Got it, thank you for making it simple. > > Overall if I try to put the tradeoffs of moving to indirect-offset > > (compared to current scheme)– > > > > Upside: > > - cqe res/flags would be intact, avoids future-headaches as you mentioned > > - short-write cases do not have to be failed in lower-layers (as > > cqe->res is there to report bytes-copied) > > I personally think it is a super bad idea to allow short asynchronous append > writes. The interface should allow the async zone append write to proceed only > and only if it can be stuffed entirely into a single BIO which necessarilly will > be a single request on the device side. Otherwise, the application would have no > guarantees as to where a split may happen, and since this is zone append, the > next async append will not leave any hole to complete a previous short write. > This will wreak the structure of the application data. > > For the sync case, this is fine. The application can just issue a new append > write with the remaining unwritten data from the previous append write. But in > the async case, if one write == one data record (e.g. a key-value tuple for an > SSTable in an LSM tree), then allowing a short write will destroy the record: > the partial write will be garbage data that will need garbage collection... There are cases when short-write is fine, isn't it? For example I can serve only 8K write (either because of space, or because of those file limits), but application sent 12K.....iov_iter_gets truncated to 8K and the write is successful. At least that's what O_APPEND and RWF_APPEND behaves currently. But in the current scheme there is no way to report number-of-bytes copied in io-uring, so I had to fail such short-write in lower-layer (which does not know whether it is talking to io_uring or aio). Failing such short-write is perhaps fine for zone-appened, but is it fine for generic file-append? > > Downside: > > - We may not be able to use RWF_APPEND, and need exposing a new > > type/flag (RWF_INDIRECT_OFFSET etc.) user-space. Not sure if this > > sounds outrageous, but is it OK to have uring-only flag which can be > > combined with RWF_APPEND? > > Why ? Where is the problem ? O_APPEND/RWF_APPEND is currently meaningless for > raw block device accesses. We could certainly define a meaning for these in the > context of zoned block devices. But application using O_APPEND/RWF_APPEND does not pass a pointer to be updated by kernel. While in kernel we would expect that, and may start writing something which is not a pointer. > I already commented on the need for first defining an interface (flags etc) and > its semantic (e.g. do we allow short zone append or not ? What happens for > regular files ? etc). Did you read my comment ? We really need to first agree on > something to clarify what needs to be done. I read and was planning to respond, sorry. But it seemed important to get the clarity on the uring-interface, as this seems to decide how this whole thing looks like (to application and to lower layers as well). > > - Expensive compared to sending results in cqe itself. But I agree > > that this may not be major, and only for one type of write. > > > > > > > -- > Damien Le Moal > Western Digital Research -- Joshi
next prev parent reply index Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <CGME20200724155244epcas5p2902f57e36e490ee8772da19aa9408cdc@epcas5p2.samsung.com> 2020-07-24 15:49 ` [PATCH v4 0/6] zone-append support in io-uring and aio Kanchan Joshi [not found] ` <CGME20200724155258epcas5p1a75b926950a18cd1e6c8e7a047e6c589@epcas5p1.samsung.com> 2020-07-24 15:49 ` [PATCH v4 1/6] fs: introduce FMODE_ZONE_APPEND and IOCB_ZONE_APPEND Kanchan Joshi 2020-07-24 16:34 ` Jens Axboe 2020-07-26 15:18 ` Christoph Hellwig 2020-07-28 1:49 ` Matthew Wilcox 2020-07-28 7:26 ` Christoph Hellwig [not found] ` <CGME20200724155324epcas5p18e1d3b4402d1e4a8eca87d0b56a3fa9b@epcas5p1.samsung.com> 2020-07-24 15:49 ` [PATCH v4 2/6] fs: change ki_complete interface to support 64bit ret2 Kanchan Joshi 2020-07-26 15:18 ` Christoph Hellwig [not found] ` <CGME20200724155329epcas5p345ba6bad0b8fe18056bb4bcd26c10019@epcas5p3.samsung.com> 2020-07-24 15:49 ` [PATCH v4 3/6] uio: return status with iov truncation Kanchan Joshi [not found] ` <CGME20200724155341epcas5p15bfc55927f2abb60f19784270fe8e377@epcas5p1.samsung.com> 2020-07-24 15:49 ` [PATCH v4 4/6] block: add zone append handling for direct I/O path Kanchan Joshi 2020-07-26 15:19 ` Christoph Hellwig [not found] ` <CGME20200724155346epcas5p2cfb383fe9904a45280c6145f4c13e1b4@epcas5p2.samsung.com> 2020-07-24 15:49 ` [PATCH v4 5/6] block: enable zone-append for iov_iter of bvec type Kanchan Joshi 2020-07-26 15:20 ` Christoph Hellwig [not found] ` <CGME20200724155350epcas5p3b8f1d59eda7f8fbb38c828f692d42fd6@epcas5p3.samsung.com> 2020-07-24 15:49 ` [PATCH v4 6/6] io_uring: add support for zone-append Kanchan Joshi 2020-07-24 16:29 ` Jens Axboe 2020-07-27 19:16 ` Kanchan Joshi 2020-07-27 20:34 ` Jens Axboe 2020-07-30 16:08 ` Pavel Begunkov 2020-07-30 16:13 ` Jens Axboe 2020-07-30 16:26 ` Pavel Begunkov 2020-07-30 17:16 ` Jens Axboe 2020-07-30 17:38 ` Pavel Begunkov 2020-07-30 17:51 ` Kanchan Joshi 2020-07-30 17:54 ` Jens Axboe 2020-07-30 18:25 ` Kanchan Joshi 2020-07-31 6:42 ` Damien Le Moal 2020-07-31 6:45 ` hch 2020-07-31 6:59 ` Damien Le Moal 2020-07-31 7:58 ` Kanchan Joshi 2020-07-31 8:14 ` Damien Le Moal 2020-07-31 9:14 ` hch 2020-07-31 9:34 ` Damien Le Moal 2020-07-31 9:41 ` hch 2020-07-31 10:16 ` Damien Le Moal 2020-07-31 12:51 ` hch 2020-07-31 13:08 ` hch 2020-07-31 15:07 ` Kanchan Joshi 2020-08-05 7:35 ` Damien Le Moal 2020-08-14 8:14 ` hch 2020-08-14 8:27 ` Damien Le Moal 2020-08-14 12:04 ` hch 2020-08-14 12:20 ` Damien Le Moal 2020-09-07 7:01 ` Kanchan Joshi 2020-09-08 15:18 ` hch 2020-09-24 17:19 ` Kanchan Joshi 2020-09-25 2:52 ` Damien Le Moal 2020-09-28 18:58 ` Kanchan Joshi 2020-09-29 1:24 ` Damien Le Moal 2020-09-29 18:49 ` Kanchan Joshi 2020-07-31 9:38 ` Kanchan Joshi 2020-07-31 7:08 ` Kanchan Joshi [this message] 2020-07-30 15:57 ` Pavel Begunkov
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