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Mon, 19 Aug 2024 13:02:10 -0700 (PDT) Received: from [192.168.1.150] ([198.8.77.157]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2d3e2e6b1f6sm7772049a91.19.2024.08.19.13.02.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 19 Aug 2024 13:02:10 -0700 (PDT) Message-ID: <5d7aabe5-3988-4a85-b329-4dfe89b22582@kernel.dk> Date: Mon, 19 Aug 2024 14:02:09 -0600 Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 5/5] block: implement io_uring discard cmd To: Pavel Begunkov , Ming Lei Cc: io-uring@vger.kernel.org, Conrad Meyer , linux-block@vger.kernel.org, linux-mm@kvack.org References: <6ecd7ab3386f63f1656dc766c1b5b038ff5353c2.1723601134.git.asml.silence@gmail.com> <4d016a30-d258-4d0e-b3bc-18bf0bd48e32@kernel.dk> Content-Language: en-US From: Jens Axboe In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 8/15/24 8:16 PM, Pavel Begunkov wrote: > On 8/16/24 03:08, Ming Lei wrote: >> On Fri, Aug 16, 2024 at 02:59:49AM +0100, Pavel Begunkov wrote: >>> On 8/16/24 02:45, Ming Lei wrote: >>>> On Thu, Aug 15, 2024 at 07:24:16PM -0600, Jens Axboe wrote: >>>>> On 8/15/24 5:44 PM, Ming Lei wrote: >>>>>> On Thu, Aug 15, 2024 at 06:11:13PM +0100, Pavel Begunkov wrote: >>>>>>> On 8/15/24 15:33, Jens Axboe wrote: >>>>>>>> On 8/14/24 7:42 PM, Ming Lei wrote: >>>>>>>>> On Wed, Aug 14, 2024 at 6:46?PM Pavel Begunkov wrote: >>>>>>>>>> >>>>>>>>>> Add ->uring_cmd callback for block device files and use it to implement >>>>>>>>>> asynchronous discard. Normally, it first tries to execute the command >>>>>>>>>> from non-blocking context, which we limit to a single bio because >>>>>>>>>> otherwise one of sub-bios may need to wait for other bios, and we don't >>>>>>>>>> want to deal with partial IO. If non-blocking attempt fails, we'll retry >>>>>>>>>> it in a blocking context. >>>>>>>>>> >>>>>>>>>> Suggested-by: Conrad Meyer >>>>>>>>>> Signed-off-by: Pavel Begunkov >>>>>>>>>> --- >>>>>>>>>>     block/blk.h             |  1 + >>>>>>>>>>     block/fops.c            |  2 + >>>>>>>>>>     block/ioctl.c           | 94 +++++++++++++++++++++++++++++++++++++++++ >>>>>>>>>>     include/uapi/linux/fs.h |  2 + >>>>>>>>>>     4 files changed, 99 insertions(+) >>>>>>>>>> >>>>>>>>>> diff --git a/block/blk.h b/block/blk.h >>>>>>>>>> index e180863f918b..5178c5ba6852 100644 >>>>>>>>>> --- a/block/blk.h >>>>>>>>>> +++ b/block/blk.h >>>>>>>>>> @@ -571,6 +571,7 @@ blk_mode_t file_to_blk_mode(struct file *file); >>>>>>>>>>     int truncate_bdev_range(struct block_device *bdev, blk_mode_t mode, >>>>>>>>>>                    loff_t lstart, loff_t lend); >>>>>>>>>>     long blkdev_ioctl(struct file *file, unsigned cmd, unsigned long arg); >>>>>>>>>> +int blkdev_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags); >>>>>>>>>>     long compat_blkdev_ioctl(struct file *file, unsigned cmd, unsigned long arg); >>>>>>>>>> >>>>>>>>>>     extern const struct address_space_operations def_blk_aops; >>>>>>>>>> diff --git a/block/fops.c b/block/fops.c >>>>>>>>>> index 9825c1713a49..8154b10b5abf 100644 >>>>>>>>>> --- a/block/fops.c >>>>>>>>>> +++ b/block/fops.c >>>>>>>>>> @@ -17,6 +17,7 @@ >>>>>>>>>>     #include >>>>>>>>>>     #include >>>>>>>>>>     #include >>>>>>>>>> +#include >>>>>>>>>>     #include "blk.h" >>>>>>>>>> >>>>>>>>>>     static inline struct inode *bdev_file_inode(struct file *file) >>>>>>>>>> @@ -873,6 +874,7 @@ const struct file_operations def_blk_fops = { >>>>>>>>>>            .splice_read    = filemap_splice_read, >>>>>>>>>>            .splice_write   = iter_file_splice_write, >>>>>>>>>>            .fallocate      = blkdev_fallocate, >>>>>>>>>> +       .uring_cmd      = blkdev_uring_cmd, >>>>>>>>> >>>>>>>>> Just be curious, we have IORING_OP_FALLOCATE already for sending >>>>>>>>> discard to block device, why is .uring_cmd added for this purpose? >>>>>>> >>>>>>> Which is a good question, I haven't thought about it, but I tend to >>>>>>> agree with Jens. Because vfs_fallocate is created synchronous >>>>>>> IORING_OP_FALLOCATE is slow for anything but pretty large requests. >>>>>>> Probably can be patched up, which would  involve changing the >>>>>>> fops->fallocate protot, but I'm not sure async there makes sense >>>>>>> outside of bdev (?), and cmd approach is simpler, can be made >>>>>>> somewhat more efficient (1 less layer in the way), and it's not >>>>>>> really something completely new since we have it in ioctl. >>>>>> >>>>>> Yeah, we have ioctl(DISCARD), which acquires filemap_invalidate_lock, >>>>>> same with blkdev_fallocate(). >>>>>> >>>>>> But this patch drops this exclusive lock, so it becomes async friendly, >>>>>> but may cause stale page cache. However, if the lock is required, it can't >>>>>> be efficient anymore and io-wq may be inevitable, :-) >>>>> >>>>> If you want to grab the lock, you can still opportunistically grab it. >>>>> For (by far) the common case, you'll get it, and you can still do it >>>>> inline. >>>> >>>> If the lock is grabbed in the whole cmd lifetime, it is basically one sync >>>> interface cause there is at most one async discard cmd in-flight for each >>>> device. >>>> >>>> Meantime the handling has to move to io-wq for avoiding to block current >>>> context, the interface becomes same with IORING_OP_FALLOCATE? >>> >>> Right, and agree that we can't trylock because we'd need to keep it >>> locked until IO completes, at least the sync versions does that. >>> >>> But I think *invalidate_pages() in the patch should be enough. That's >>> what the write path does, so it shouldn't cause any problem to the >>> kernel. As for user space, that'd be more relaxed than the ioctl, >>> just as writes are, so nothing new to the user. I hope someone with >>> better filemap understanding can confirm it (or not). >> >> I may not be familiar with filemap enough, but looks *invalidate_pages() >> is only for removing pages from the page cache range, and the lock is added >> for preventing new page cache read from being started, so stale data read >> can be avoided when DISCARD is in-progress. > > Sounds like it, but the point is it's the same data race for the > user as if it would've had a write in progress. Right, which is why it should not matter. I think it's pretty silly to take the sync implementation as gospel here, assuming that the original author knew what they were doing in full detail. It just needs proper documenting. -- Jens Axboe