From: Hao Xu <[email protected]>
To: Christian Brauner <[email protected]>,
[email protected], Dave Chinner <[email protected]>,
Jens Axboe <[email protected]>
Cc: [email protected],
Dominique Martinet <[email protected]>,
Pavel Begunkov <[email protected]>,
Alexander Viro <[email protected]>,
Stefan Roesch <[email protected]>, Clay Harris <[email protected]>,
[email protected], Wanpeng Li <[email protected]>
Subject: Re: [PATCH 3/5] io_uring: add support for getdents
Date: Mon, 31 Jul 2023 02:02:25 +0800 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <20230727-salbe-kurvigen-31b410c07bb9@brauner>
Hi Christian,
On 7/27/23 22:27, Christian Brauner wrote:
> On Thu, Jul 27, 2023 at 07:51:19PM +0800, Hao Xu wrote:
>> On 7/26/23 23:00, Christian Brauner wrote:
>>> On Tue, Jul 18, 2023 at 09:21:10PM +0800, Hao Xu wrote:
>>>> From: Hao Xu <[email protected]>
>>>>
>>>> This add support for getdents64 to io_uring, acting exactly like the
>>>> syscall: the directory is iterated from it's current's position as
>>>> stored in the file struct, and the file's position is updated exactly as
>>>> if getdents64 had been called.
>>>>
>>>> For filesystems that support NOWAIT in iterate_shared(), try to use it
>>>> first; if a user already knows the filesystem they use do not support
>>>> nowait they can force async through IOSQE_ASYNC in the sqe flags,
>>>> avoiding the need to bounce back through a useless EAGAIN return.
>>>>
>>>> Co-developed-by: Dominique Martinet <[email protected]>
>>>> Signed-off-by: Dominique Martinet <[email protected]>
>>>> Signed-off-by: Hao Xu <[email protected]>
>>>> ---
>>>> include/uapi/linux/io_uring.h | 7 +++++
>>>> io_uring/fs.c | 55 +++++++++++++++++++++++++++++++++++
>>>> io_uring/fs.h | 3 ++
>>>> io_uring/opdef.c | 8 +++++
>>>> 4 files changed, 73 insertions(+)
>>>>
>>>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
>>>> index 36f9c73082de..b200b2600622 100644
>>>> --- a/include/uapi/linux/io_uring.h
>>>> +++ b/include/uapi/linux/io_uring.h
>>>> @@ -65,6 +65,7 @@ struct io_uring_sqe {
>>>> __u32 xattr_flags;
>>>> __u32 msg_ring_flags;
>>>> __u32 uring_cmd_flags;
>>>> + __u32 getdents_flags;
>>>> };
>>>> __u64 user_data; /* data to be passed back at completion time */
>>>> /* pack this to avoid bogus arm OABI complaints */
>>>> @@ -235,6 +236,7 @@ enum io_uring_op {
>>>> IORING_OP_URING_CMD,
>>>> IORING_OP_SEND_ZC,
>>>> IORING_OP_SENDMSG_ZC,
>>>> + IORING_OP_GETDENTS,
>>>> /* this goes last, obviously */
>>>> IORING_OP_LAST,
>>>> @@ -273,6 +275,11 @@ enum io_uring_op {
>>>> */
>>>> #define SPLICE_F_FD_IN_FIXED (1U << 31) /* the last bit of __u32 */
>>>> +/*
>>>> + * sqe->getdents_flags
>>>> + */
>>>> +#define IORING_GETDENTS_REWIND (1U << 0)
>>>> +
>>>> /*
>>>> * POLL_ADD flags. Note that since sqe->poll_events is the flag space, the
>>>> * command flags for POLL_ADD are stored in sqe->len.
>>>> diff --git a/io_uring/fs.c b/io_uring/fs.c
>>>> index f6a69a549fd4..480f25677fed 100644
>>>> --- a/io_uring/fs.c
>>>> +++ b/io_uring/fs.c
>>>> @@ -47,6 +47,13 @@ struct io_link {
>>>> int flags;
>>>> };
>>>> +struct io_getdents {
>>>> + struct file *file;
>>>> + struct linux_dirent64 __user *dirent;
>>>> + unsigned int count;
>>>> + int flags;
>>>> +};
>>>> +
>>>> int io_renameat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>>> {
>>>> struct io_rename *ren = io_kiocb_to_cmd(req, struct io_rename);
>>>> @@ -291,3 +298,51 @@ void io_link_cleanup(struct io_kiocb *req)
>>>> putname(sl->oldpath);
>>>> putname(sl->newpath);
>>>> }
>>>> +
>>>> +int io_getdents_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>>> +{
>>>> + struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents);
>>>> +
>>>> + if (READ_ONCE(sqe->off) != 0)
>>>> + return -EINVAL;
>>>> +
>>>> + gd->dirent = u64_to_user_ptr(READ_ONCE(sqe->addr));
>>>> + gd->count = READ_ONCE(sqe->len);
>>>> +
>>>> + return 0;
>>>> +}
>>>> +
>>>> +int io_getdents(struct io_kiocb *req, unsigned int issue_flags)
>>>> +{
>>>> + struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents);
>>>> + struct file *file = req->file;
>>>> + unsigned long getdents_flags = 0;
>>>> + bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
>>>
>>> Hm, I'm not sure what exactly the rules are for IO_URING_F_NONBLOCK.
>>> But to point this out:
>>>
>>> vfs_getdents()
>>> -> iterate_dir()
>>> {
>>> if (shared)
>>> res = down_read_killable(&inode->i_rwsem);
>>> else
>>> res = down_write_killable(&inode->i_rwsem);
>>> }
>>>
>>> which means you can still end up sleeping here before you go into a
>>> filesystem that does actually support non-waiting getdents. So if you
>>> have concurrent operations that grab inode lock (touch, mkdir etc) you
>>> can end up sleeping here.
>>>
>>> Is that intentional or an oversight? If the former can someone please
>>> explain the rules and why it's fine in this case?
>>
>> I actually saw this semaphore, and there is another xfs lock in
>> file_accessed
>> --> touch_atime
>> --> inode_update_time
>> --> inode->i_op->update_time == xfs_vn_update_time
>>
>> Forgot to point them out in the cover-letter..., I didn't modify them
>> since I'm not very sure about if we should do so, and I saw Stefan's
>> patchset didn't modify them too.
>>
>> My personnal thinking is we should apply trylock logic for this
>> inode->i_rwsem. For xfs lock in touch_atime, we should do that since it
>> doesn't make sense to rollback all the stuff while we are almost at the
>> end of getdents because of a lock.
>
> That manoeuvres around the problem. Which I'm slightly more sensitive
> too as this review is a rather expensive one.
>
> Plus, it seems fixable in at least two ways:
>
> For both we need to be able to tell the filesystem that a nowait atime
> update is requested. Simple thing seems to me to add a S_NOWAIT flag to
> file_time_flags and passing that via i_op->update_time() which already
> has a flag argument. That would likely also help kiocb_modified().
>
> file_accessed()
> -> touch_atime()
> -> inode_update_time()
> -> i_op->update_time == xfs_vn_update_time()
>
> Then we have two options afaict:
>
> (1) best-effort atime update
>
> file_accessed() already has the builtin assumption that updating atime
> might fail for other reasons - see the comment in there. So it is
> somewhat best-effort already.
>
> (2) move atime update before calling into filesystem
>
> If we want to be sure that access time is updated when a readdir request
> is issued through io_uring then we need to have file_accessed() give a
> return value and expose a new helper for io_uring or modify
> vfs_getdents() to do something like:
>
> vfs_getdents()
> {
> if (nowait)
> down_read_trylock()
>
> if (!IS_DEADDIR(inode)) {
> ret = file_accessed(file);
> if (ret == -EAGAIN)
> goto out_unlock;
>
> f_op->iterate_shared()
> }
> }
>
> It's not unprecedented to do update atime before the actual operation
> has been done afaict. That's already the case in xfs_file_write_checks()
> which is called before anything is written. So that seems ok.
I'm not familiar with this part(the time update), I guess we should
revert the updated time if we succeed to do file_accessed(file) but
fail somewhere later in f_op->iterate_shared()? Or is it definitely
counted as an "access" as long as we start to call getdents to a file?
Thanks,
Hao
>
> Does any of these two options work for the xfs maintainers and Jens?
next prev parent reply other threads:[~2023-07-30 18:02 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-18 13:21 [PATCH v4 0/5] io_uring getdents Hao Xu
2023-07-18 13:21 ` [PATCH 1/5] fs: split off vfs_getdents function of getdents64 syscall Hao Xu
2023-07-18 13:21 ` [PATCH 2/5] vfs_getdents/struct dir_context: add flags field Hao Xu
2023-07-18 13:21 ` [PATCH 3/5] io_uring: add support for getdents Hao Xu
2023-07-19 8:56 ` Hao Xu
2023-07-26 15:00 ` Christian Brauner
2023-07-27 11:51 ` Hao Xu
2023-07-27 14:27 ` Christian Brauner
2023-07-27 15:12 ` Pavel Begunkov
2023-07-27 15:52 ` Christian Brauner
2023-07-27 16:17 ` Pavel Begunkov
2023-07-27 16:28 ` Christian Brauner
2023-07-31 1:58 ` Dave Chinner
2023-07-31 7:34 ` Hao Xu
2023-07-31 7:50 ` Christian Brauner
2023-07-31 7:40 ` Christian Brauner
2023-07-30 18:02 ` Hao Xu [this message]
2023-07-31 8:18 ` Christian Brauner
2023-07-31 9:31 ` Hao Xu
2023-07-31 1:33 ` Dave Chinner
2023-07-31 8:13 ` Christian Brauner
2023-07-31 15:26 ` Darrick J. Wong
2023-07-31 22:18 ` Dave Chinner
2023-08-01 0:28 ` Jens Axboe
2023-08-01 0:47 ` Matthew Wilcox
2023-08-01 0:49 ` Jens Axboe
2023-08-01 1:01 ` Matthew Wilcox
2023-08-01 7:00 ` Christian Brauner
2023-08-01 6:59 ` Christian Brauner
2023-08-01 7:17 ` Christian Brauner
2023-08-08 4:34 ` Hao Xu
2023-08-08 5:18 ` Hao Xu
2023-08-08 9:33 ` Hao Xu
2023-08-08 22:55 ` Jens Axboe
2023-08-01 18:39 ` Hao Xu
2023-07-18 13:21 ` [PATCH 4/5] xfs: add NOWAIT semantics for readdir Hao Xu
2023-07-19 2:35 ` kernel test robot
2023-07-18 13:21 ` [PATCH RFC 5/5] disable fixed file for io_uring getdents for now Hao Xu
2023-07-26 14:23 ` Christian Brauner
2023-07-27 12:09 ` Hao Xu
2023-07-19 6:04 ` [PATCH v4 0/5] io_uring getdents Christian Brauner
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