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Subject: Re: [PATCH v3 2/2] io_uring: add support for IORING_OP_GETDENTS
From:   Pavel Begunkov <asml.silence@gmail.com>
To:     Lennert Buytenhek <buytenh@wantstofly.org>,
        Jens Axboe <axboe@kernel.dk>,
        Al Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        io-uring@vger.kernel.org
Cc:     David Laight <David.Laight@aculab.com>,
        Matthew Wilcox <willy@infradead.org>
References: <20210218122640.GA334506@wantstofly.org>
 <20210218122755.GC334506@wantstofly.org>
 <9a6fb59b-be85-c36b-3c83-26cff37bcb87@gmail.com>
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Message-ID: <8b675c3d-3d25-aaca-7796-e02bba2da01a@gmail.com>
Date:   Fri, 19 Feb 2021 12:10:02 +0000
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On 19/02/2021 12:05, Pavel Begunkov wrote:
> On 18/02/2021 12:27, Lennert Buytenhek wrote:
>> IORING_OP_GETDENTS behaves much like getdents64(2) and takes the same
>> arguments, but with a small twist: it takes an additional offset
>> argument, and reading from the specified directory starts at the given
>> offset.
>>
>> For the first IORING_OP_GETDENTS call on a directory, the offset
>> parameter can be set to zero, and for subsequent calls, it can be
>> set to the ->d_off field of the last struct linux_dirent64 returned
>> by the previous IORING_OP_GETDENTS call.
>>
>> Internally, if necessary, IORING_OP_GETDENTS will vfs_llseek() to
>> the right directory position before calling vfs_getdents().
>>
>> IORING_OP_GETDENTS may or may not update the specified directory's
>> file offset, and the file offset should not be relied upon having
>> any particular value during or after an IORING_OP_GETDENTS call.
>>
>> Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
>> ---
>>  fs/io_uring.c                 | 73 +++++++++++++++++++++++++++++++++++
>>  include/uapi/linux/io_uring.h |  1 +
>>  2 files changed, 74 insertions(+)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 056bd4c90ade..6853bf48369a 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -635,6 +635,13 @@ struct io_mkdir {
>>  	struct filename			*filename;
>>  };
>>  
> [...]
>> +static int io_getdents(struct io_kiocb *req, unsigned int issue_flags)
>> +{
>> +	struct io_getdents *getdents = &req->getdents;
>> +	bool pos_unlock = false;
>> +	int ret = 0;
>> +
>> +	/* getdents always requires a blocking context */
>> +	if (issue_flags & IO_URING_F_NONBLOCK)
>> +		return -EAGAIN;
>> +
>> +	/* for vfs_llseek and to serialize ->iterate_shared() on this file */
>> +	if (file_count(req->file) > 1) {
> 
> Looks racy, is it safe? E.g. can be concurrently dupped and used, or just
> several similar IORING_OP_GETDENTS requests.
> 
>> +		pos_unlock = true;
>> +		mutex_lock(&req->file->f_pos_lock);
>> +	}
>> +
>> +	if (req->file->f_pos != getdents->pos) {
>> +		loff_t res = vfs_llseek(req->file, getdents->pos, SEEK_SET);
> 
> I may be missing the previous discussions, but can this ever become
> stateless, like passing an offset? Including readdir.c and beyond. 

I mean without those seeks. An emulation would look like rewinding
pos back after vfs_getdents, though might be awful on performance. 

> 
>> +		if (res < 0)
>> +			ret = res;
>> +	}
>> +
>> +	if (ret == 0) {
>> +		ret = vfs_getdents(req->file, getdents->dirent,
>> +				   getdents->count);
>> +	}
>> +
>> +	if (pos_unlock)
>> +		mutex_unlock(&req->file->f_pos_lock);
>> +
>> +	if (ret < 0) {
>> +		if (ret == -ERESTARTSYS)
>> +			ret = -EINTR;
>> +		req_set_fail_links(req);
>> +	}
>> +	io_req_complete(req, ret);
>> +	return 0;
>> +}
> [...]
> 

-- 
Pavel Begunkov