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Date: Sun, 24 May 2020 11:12:47 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Trond Myklebust <trondmy@...merspace.com>,
"io-uring@...r.kernel.org" <io-uring@...r.kernel.org>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/12] mm: support async buffered reads in
generic_file_buffered_read()
On 5/24/20 11:11 AM, Trond Myklebust wrote:
> On Sun, 2020-05-24 at 10:40 -0600, Jens Axboe wrote:
>> On 5/24/20 10:30 AM, Jens Axboe wrote:
>>> On 5/24/20 8:05 AM, Trond Myklebust wrote:
>>>> On Sat, 2020-05-23 at 12:57 -0600, Jens Axboe wrote:
>>>>> Use the async page locking infrastructure, if IOCB_WAITQ is set
>>>>> in
>>>>> the
>>>>> passed in iocb. The caller must expect an -EIOCBQUEUED return
>>>>> value,
>>>>> which means that IO is started but not done yet. This is
>>>>> similar to
>>>>> how
>>>>> O_DIRECT signals the same operation. Once the callback is
>>>>> received by
>>>>> the caller for IO completion, the caller must retry the
>>>>> operation.
>>>>>
>>>>> Signed-off-by: Jens Axboe <axboe@...nel.dk>
>>>>> ---
>>>>> mm/filemap.c | 33 ++++++++++++++++++++++++++-------
>>>>> 1 file changed, 26 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>>> index c746541b1d49..a3b86c9acdc8 100644
>>>>> --- a/mm/filemap.c
>>>>> +++ b/mm/filemap.c
>>>>> @@ -1219,6 +1219,14 @@ static int
>>>>> __wait_on_page_locked_async(struct
>>>>> page *page,
>>>>> return ret;
>>>>> }
>>>>>
>>>>> +static int wait_on_page_locked_async(struct page *page,
>>>>> + struct wait_page_queue
>>>>> *wait)
>>>>> +{
>>>>> + if (!PageLocked(page))
>>>>> + return 0;
>>>>> + return __wait_on_page_locked_async(compound_head(page),
>>>>> wait,
>>>>> false);
>>>>> +}
>>>>> +
>>>>> /**
>>>>> * put_and_wait_on_page_locked - Drop a reference and wait for
>>>>> it to
>>>>> be unlocked
>>>>> * @page: The page to wait for.
>>>>> @@ -2058,17 +2066,25 @@ static ssize_t
>>>>> generic_file_buffered_read(struct kiocb *iocb,
>>>>> index, last_index -
>>>>> index);
>>>>> }
>>>>> if (!PageUptodate(page)) {
>>>>> - if (iocb->ki_flags & IOCB_NOWAIT) {
>>>>> - put_page(page);
>>>>> - goto would_block;
>>>>> - }
>>>>> -
>>>>> /*
>>>>> * See comment in do_read_cache_page on
>>>>> why
>>>>> * wait_on_page_locked is used to avoid
>>>>> unnecessarily
>>>>> * serialisations and why it's safe.
>>>>> */
>>>>> - error =
>>>>> wait_on_page_locked_killable(page);
>>>>> + if (iocb->ki_flags & IOCB_WAITQ) {
>>>>> + if (written) {
>>>>> + put_page(page);
>>>>> + goto out;
>>>>> + }
>>>>> + error =
>>>>> wait_on_page_locked_async(page,
>>>>> +
>>>>> iocb-
>>>>>> private);
>>>>
>>>> If it is being used in 'generic_file_buffered_read()' as storage
>>>> for a
>>>> wait queue, then it is hard to consider this a 'private' field.
>>>
>>> private isn't the prettiest, and in fact this one in particular is
>>> a bit
>>> of a mess. It's not clear if it's caller or callee owned. It's
>>> generally
>>> not used, outside of the old usb gadget code, iomap O_DIRECT, and
>>> ocfs2.
>>> With FMODE_BUF_RASYNC, the fs obviously can't set it if it uses
>>> ->private for buffered IO.
>>>
>>>> Perhaps either rename and add type checking, or else add a
>>>> separate
>>>> field altogether to struct kiocb?
>>>
>>> I'd hate to add a new field and increase the size of the kiocb...
>>> One
>>> alternative is to do:
>>>
>>> union {
>>> void *private;
>>> struct wait_page_queue *ki_waitq;
>>> };
>>>
>>> and still use IOCB_WAITQ to say that ->ki_waitq is valid.
>>>
>>> There's also 4 bytes of padding in the kiocb struct. And some
>>> fields are
>>> only used for O_DIRECT as well, eg ->ki_cookie which is just used
>>> for
>>> polled O_DIRECT. So we could also do:
>>>
>>> union {
>>> unsigned int ki_cookie;
>>> struct wait_page_queue *ki_waitq;
>>> };
>>>
>>> and still not grow the kiocb. How about we go with this approach,
>>> and
>>> also add:
>>>
>>> if (kiocb->ki_flags & IOCB_HIPRI)
>>> return -EOPNOTSUPP;
>>>
>>> to kiocb_wait_page_queue_init() to make sure that this combination
>>> isn't
>>> valid?
>>
>> Here's the incremental, which is spread over 3 patches. I think this
>> one
>> makes sense, as polled IO doesn't support buffered IO. And because
>> doing
>> an async callback for completion is not how polled IO operates
>> anyway,
>> so even if buffered IO supported it, we'd not use the callback for
>> polled IO anyway. kiocb_wait_page_queue_init() checks and backs this
>> up.
>>
>>
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 0ef5f5973b1c..f7b1eb765c6e 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -317,7 +317,7 @@ enum rw_hint {
>> #define IOCB_SYNC (1 << 5)
>> #define IOCB_WRITE (1 << 6)
>> #define IOCB_NOWAIT (1 << 7)
>> -/* iocb->private holds wait_page_async struct */
>> +/* iocb->ki_waitq is valid */
>> #define IOCB_WAITQ (1 << 8)
>>
>> struct kiocb {
>> @@ -332,7 +332,10 @@ struct kiocb {
>> int ki_flags;
>> u16 ki_hint;
>> u16 ki_ioprio; /* See linux/ioprio.h */
>> - unsigned int ki_cookie; /* for ->iopoll */
>> + union {
>> + unsigned int ki_cookie; /* for ->iopoll */
>> + struct wait_page_queue *ki_waitq; /* for async
>> buffered IO */
>> + };
>>
>> randomized_struct_fields_end
>> };
>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>> index def58de92053..8b65420410ee 100644
>> --- a/include/linux/pagemap.h
>> +++ b/include/linux/pagemap.h
>> @@ -498,13 +498,16 @@ static inline int
>> kiocb_wait_page_queue_init(struct kiocb *kiocb,
>> wait_queue_func_t func,
>> void *data)
>> {
>> + /* Can't support async wakeup with polled IO */
>> + if (kiocb->ki_flags & IOCB_HIPRI)
>> + return -EINVAL;
>> if (kiocb->ki_filp->f_mode & FMODE_BUF_RASYNC) {
>> wait->wait.func = func;
>> wait->wait.private = data;
>> wait->wait.flags = 0;
>> INIT_LIST_HEAD(&wait->wait.entry);
>> kiocb->ki_flags |= IOCB_WAITQ;
>> - kiocb->private = wait;
>> + kiocb->ki_waitq = wait;
>> return 0;
>> }
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index a3b86c9acdc8..18022de7dc33 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -2077,7 +2077,7 @@ static ssize_t
>> generic_file_buffered_read(struct kiocb *iocb,
>> goto out;
>> }
>> error = wait_on_page_locked_async(page,
>> - iocb-
>>> private);
>> + iocb-
>>> ki_waitq);
>> } else {
>> if (iocb->ki_flags & IOCB_NOWAIT) {
>> put_page(page);
>> @@ -2173,7 +2173,7 @@ static ssize_t
>> generic_file_buffered_read(struct kiocb *iocb,
>> page_not_up_to_date:
>> /* Get exclusive access to the page ... */
>> if (iocb->ki_flags & IOCB_WAITQ)
>> - error = lock_page_async(page, iocb->private);
>> + error = lock_page_async(page, iocb->ki_waitq);
>> else
>> error = lock_page_killable(page);
>> if (unlikely(error))
>
> Ack. That seems cleaner to me.
I agree, this is better. Ran it through the testing and updated the
series accordingly:
https://git.kernel.dk/cgit/linux-block/log/?h=async-buffered.4
--
Jens Axboe
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