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From: Jens Axboe <axboe@kernel.dk>
To: Roman Penyaev <rpenyaev@suse.de>
Cc: linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
	linux-block@vger.kernel.org, linux-arch@vger.kernel.org,
	hch@lst.de, jmoyer@redhat.com, avi@scylladb.com,
	linux-block-owner@vger.kernel.org
Subject: Re: [PATCH 05/15] Add io_uring IO interface
Date: Thu, 17 Jan 2019 07:54:43 -0700
Message-ID: <c3d902f9-06f8-9e9e-4dcf-ad09e4e291ae@kernel.dk> (raw)
In-Reply-To: <24a609aa05936eb2380f93487be8736c@suse.de>

On 1/17/19 7:34 AM, Roman Penyaev wrote:
> On 2019-01-17 14:54, Jens Axboe wrote:
>> On 1/17/19 5:02 AM, Roman Penyaev wrote:
>>> Hi Jens,
>>>
>>> On 2019-01-16 18:49, Jens Axboe wrote:
>>>
>>> [...]
>>>
>>>> +static void *io_mem_alloc(size_t size)
>>>> +{
>>>> +	gfp_t gfp_flags = GFP_KERNEL | __GFP_ZERO | __GFP_NOWARN | 
>>>> __GFP_COMP
>>>> |
>>>> +				__GFP_NORETRY;
>>>> +
>>>> +	return (void *) __get_free_pages(gfp_flags, get_order(size));
>>>
>>> Since these pages are shared between kernel and userspace, do we need
>>> to care about d-cache aliasing on armv6 (or other "strange" archs
>>> which I've never seen) with vivt or vipt cpu caches?
>>>
>>> E.g. vmalloc_user() targets this problem by aligning kernel address
>>> on SHMLBA, so no flush_dcache_page() is required.
>>
>> I'm honestly not sure, it'd be trivial enough to stick a
>> flush_dcache_page() into the few areas we'd need it. The rings are
>> already page (SHMLBA) aligned.
> 
> For arm SHMLBA is not a page, it is 4x page.  So for userspace vaddr
> which mmap() returns is aligned, but for kernel not.  So indeed
> flush_dcache_page() should be used.

Oh indeed, my bad.

> The other question which I can't answer myself is the order of
> flush_dcache_page() and smp_wmb().  Does flush_scache_page() implies
> flush of the cpu write buffer?   Or firstly smp_wmb() should be done
> in order to flush everything to cache.  Here is what arm spec says
> about write-back cache:
> 
> "Writes that miss in the cache are placed in the write buffer and
> appear on the AMBA ASB interface. The CPU continues execution as
> soon as the write is placed in the write buffer."
> 
> So if you firstly do flush_dcache_page() will it flush write buffer?
> Because it seems that firstly smp_wmb() and then flush_dcache_page(),
> or I am going mad?

I don't think you're going mad! We'd first need smp_wmb() to order the
writes, then the flush_dcache_page(). For filling the CQ ring, we'd also
need to flush the page the cqe belongs to.

Question is if we care enough about performance on vivt to do something
about that. I know what my answer will be... If others care, they can
incrementally improve upon that.

-- 
Jens Axboe


  reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-16 17:49 [PATCHSET v5] " Jens Axboe
2019-01-16 17:49 ` [PATCH 01/15] fs: add an iopoll method to struct file_operations Jens Axboe
2019-01-16 17:49 ` [PATCH 02/15] block: wire up block device iopoll method Jens Axboe
2019-01-16 17:49 ` [PATCH 03/15] block: add bio_set_polled() helper Jens Axboe
2019-01-16 17:49 ` [PATCH 04/15] iomap: wire up the iopoll method Jens Axboe
2019-01-16 17:49 ` [PATCH 05/15] Add io_uring IO interface Jens Axboe
2019-01-17 12:02   ` Roman Penyaev
2019-01-17 13:54     ` Jens Axboe
2019-01-17 14:34       ` Roman Penyaev
2019-01-17 14:54         ` Jens Axboe [this message]
2019-01-17 15:19           ` Roman Penyaev
2019-01-17 12:48   ` Roman Penyaev
2019-01-17 14:01     ` Jens Axboe
2019-01-17 20:03       ` Jeff Moyer
2019-01-17 20:09         ` Jens Axboe
2019-01-17 20:14           ` Jens Axboe
2019-01-17 20:50             ` Jeff Moyer
2019-01-17 20:53               ` Jens Axboe
2019-01-17 21:02                 ` Jeff Moyer
2019-01-17 21:17                   ` Jens Axboe
2019-01-17 21:21                     ` Jeff Moyer
2019-01-17 21:27                       ` Jens Axboe
2019-01-18  8:23               ` Roman Penyaev
2019-01-16 17:49 ` [PATCH 06/15] io_uring: add fsync support Jens Axboe
2019-01-16 17:49 ` [PATCH 07/15] io_uring: support for IO polling Jens Axboe
2019-01-16 17:49 ` [PATCH 08/15] fs: add fget_many() and fput_many() Jens Axboe
2019-01-16 17:49 ` [PATCH 09/15] io_uring: use fget/fput_many() for file references Jens Axboe
2019-01-16 17:49 ` [PATCH 10/15] io_uring: batch io_kiocb allocation Jens Axboe
2019-01-16 17:49 ` [PATCH 11/15] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-01-16 17:50 ` [PATCH 12/15] io_uring: add support for pre-mapped user IO buffers Jens Axboe
2019-01-16 20:53   ` Dave Chinner
2019-01-16 21:20     ` Jens Axboe
2019-01-16 22:09       ` Dave Chinner
2019-01-16 22:21         ` Jens Axboe
2019-01-16 23:09           ` Dave Chinner
2019-01-16 23:17             ` Jens Axboe
2019-01-16 22:13       ` Jens Axboe
2019-01-16 17:50 ` [PATCH 13/15] io_uring: add submission polling Jens Axboe
2019-01-16 17:50 ` [PATCH 14/15] io_uring: add file registration Jens Axboe
2019-01-16 17:50 ` [PATCH 15/15] io_uring: add io_uring_event cache hit information Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2019-01-10  2:43 [PATCHSET v2] io_uring IO interface Jens Axboe
2019-01-10  2:43 ` [PATCH 05/15] Add " Jens Axboe
2019-01-11 18:19   ` Martin K. Petersen
2019-01-11 18:34     ` Jens Axboe
2019-01-13 16:22       ` Jens Axboe
2019-01-15 17:31         ` Martin K. Petersen

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