Re: [RFCv3 4/4] ext4: Move to shared iolock even without dioread_nolock mount opt

[Ritesh Harjani <riteshh@linux.ibm.com>]

On 11/26/19 4:21 PM, Ritesh Harjani wrote:
> Hello Jan,
> 
> Sorry about getting a little late on this.
> 
> On 11/20/19 8:02 PM, Jan Kara wrote:
>> On Wed 20-11-19 10:30:24, Ritesh Harjani wrote:
>>> We were using shared locking only in case of dioread_nolock
>>> mount option in case of DIO overwrites. This mount condition
>>> is not needed anymore with current code, since:-
>>>
>>> 1. No race between buffered writes & DIO overwrites.
>>> Since buffIO writes takes exclusive locks & DIO overwrites
>>> will take share locking. Also DIO path will make sure
>>> to flush and wait for any dirty page cache data.
>>>
>>> 2. No race between buffered reads & DIO overwrites, since there
>>> is no block allocation that is possible with DIO overwrites.
>>> So no stale data exposure should happen. Same is the case
>>> between DIO reads & DIO overwrites.
>>>
>>> 3. Also other paths like truncate is protected,
>>> since we wait there for any DIO in flight to be over.
>>>
>>> 4. In case of buffIO writes followed by DIO reads:
>>> Since here also we take exclusive locks in ext4_write_begin/end().
>>> There is no risk of exposing any stale data in this case.
>>> Since after ext4_write_end, iomap_dio_rw() will wait to flush &
>>> wait for any dirty page cache data.
>>>
>>> Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
>>
>> There's one more case to consider here as I mentioned in [1]. There 
>> can be
> 
> Yes, I should have mentioned about this in cover letter and about my
> thoughts on that.
> I was of the opinion that since the race is already existing
> and it may not be caused due to this patch, so we should handle that in 
> incremental fashion and as a separate patch series after this one.
> Let me know your thoughts on above.
> 
> Also, I wanted to have some more discussions on this race before
> making the changes.
> But nevertheless, it's the right time to discuss those changes here.
> 
>> mmap write instantiating dirty page and then someone starting writeback
>> against that page while DIO read is running still theoretically 
>> leading to
>> stale data exposure. Now this patch does not have influence on that race
>> but:
> 
> Yes, agreed.
> 
>>
>> 1) We need to close the race mentioned above. Maybe we could do that by
>> proactively allocating unwritten blocks for a page being faulted when 
>> there
>> is direct IO running against the file - the one who fills holes through
>> mmap write while direct IO is running on the file deserves to suffer the
>> performance penalty...
> 
> I was giving this a thought. So even if we try to penalize mmap
> write as you mentioned above, what I am not sure about it, is that, how 
> can we reliably detect that the DIO is in progress?
> 
> Say even if we try to check for atomic_read(&inode->i_dio_count) in mmap
> ext4_page_mkwrite path, it cannot be reliable unless there is some sort 
> of a lock protection, no?
> Because after the check the DIO can still snoop in, right?


IIRC, we had some discussion around this at [1] last time.
IIUC, you were mentioning to always using unwritten extents
as ->get_block in ext4_page_mkwrite.
And in ext4_writepages(), we replace 'ext4_should_dioread_nolock()' 
check with 'is there any DIO in flight'.

It was discussed to do that check reliably we should have all pages
locked for writeback. But how does that ensure that DIO is not currently 
in flight? One such case could be:-
It may still happen that the check to filemap_write_and_wait_range()
from DIO (iomap_dio_rw) got completed and before calling 
inode_dio_begin() a context switch happens.
And in parallel we got the page fault which somehow also resulted into 
writeback of pages calling ext4_writepages(). Here when we checked for
'is DIO in progress' after making sure all the writeback pages are 
locked, we still say may miss the reliable check if the context switch 
back to the DIO process happens right. Am I missing anything?

1. Is there any lock guarantee which I am overlooking here?

2. Do you think we should use some other lock to provide the guarantee 
between page_mkwrite & DIO read?

3. What if we always go via unwritten blocks in ext4_writepages too?
hmm, but I am not sure if should really do this today as there are some
known xfstests failures for blocksize < pagesize with dioread_nolock 
path right.
Although those problems I have mostly observed with 1K blocksize & 4K
pagesize on x86 platform.


[1] 
https://lore.kernel.org/linux-ext4/20190926134726.GA28555@quack2.suse.cz/


> 
> 
> 2. Also what about the delalloc opt. in that case? Even for delalloc
> should we go ahead and allocate the unwritten blocks? That may even need
> to start/stop the journal which could add more latency, no?
> 

One thing by always using unwritten blocks in ext4_page_mkwrite is
that the stale data exposure problem (from DIO Read) would go away.
So one small thing to note would be that it will incur some additional 
latency for delalloc too, but that is anyway there today in nodelalloc
or low disk space.

What I was thinking is that, whether there is a reliable way of directly 
detecting a "whether DIO is in flight" in ext4_page_mkwrite() function.
If yes, then we should fallback to unwritten blocks allocation path
in ext4_page_mkwrite(). And no other changes would be needed. Right?


> 
>>
>> 2) After this patch there's no point in having dioread_nolock at all 
>> so we
>> can just make that mount option no-op and drop all the precautions 
>> from the
>> buffered IO path connected with dioread_nolock.
> 
> Yes, with some careful review we should be able to drop those
> precautions related to dioread_nolock, after getting above race fixed.
> 
> 
> 
>>
>> [1] 
>> https://lore.kernel.org/linux-ext4/20190925092339.GB23277@quack2.suse.cz
>>
>>                                 Honza
>>
>>> ---
>>>   fs/ext4/file.c | 25 +++++++++++++++++++------
>>>   1 file changed, 19 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
>>> index 18cbf9fa52c6..b97efc89cd63 100644
>>> --- a/fs/ext4/file.c
>>> +++ b/fs/ext4/file.c
>>> @@ -383,6 +383,17 @@ static const struct iomap_dio_ops 
>>> ext4_dio_write_ops = {
>>>       .end_io = ext4_dio_write_end_io,
>>>   };
>>> +static bool ext4_dio_should_shared_lock(struct inode *inode)
>>> +{
>>> +    if (!S_ISREG(inode->i_mode))
>>
>> This cannot happen for DIO so no point in checking here.
>>
>>> +        return false;
>>> +    if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
>>
>> Why this?
>>
>>> +        return false;
>>> +    if (ext4_should_journal_data(inode))
>>
>> We don't do DIO when journalling data so no point in checking here
>> (dio_supported() already checked this).
>>
>>                                 Honza
>>> +        return false;
>>> +    return true;
>>> +}
>>> +
> 
> Yes, agreed we don't need this function (ext4_dio_should_shared_lock)
> anyways.
> 
> 
>>>   /*
>>>    * The intention here is to start with shared lock acquired then 
>>> see if any
>>>    * condition requires an exclusive inode lock. If yes, then we 
>>> restart the
>>> @@ -394,8 +405,8 @@ static const struct iomap_dio_ops 
>>> ext4_dio_write_ops = {
>>>    * - For extending writes case we don't take the shared lock, since 
>>> it requires
>>>    *   updating inode i_disksize and/or orphan handling with 
>>> exclusive lock.
>>>    *
>>> - * - shared locking will only be true mostly in case of overwrites with
>>> - *   dioread_nolock mode. Otherwise we will switch to excl. iolock 
>>> mode.
>>> + * - shared locking will only be true mostly in case of overwrites.
>>> + *   Otherwise we will switch to excl. iolock mode.
>>>    */
>>>   static ssize_t ext4_dio_write_checks(struct kiocb *iocb, struct 
>>> iov_iter *from,
>>>                    unsigned int *iolock, bool *unaligned_io,
>>> @@ -433,15 +444,14 @@ static ssize_t ext4_dio_write_checks(struct 
>>> kiocb *iocb, struct iov_iter *from,
>>>           *extend = true;
>>>       /*
>>>        * Determine whether the IO operation will overwrite allocated
>>> -     * and initialized blocks. If so, check to see whether it is
>>> -     * possible to take the dioread_nolock path.
>>> +     * and initialized blocks.
>>>        *
>>>        * We need exclusive i_rwsem for changing security info
>>>        * in file_modified().
>>>        */
>>>       if (*iolock == EXT4_IOLOCK_SHARED &&
>>>           (!IS_NOSEC(inode) || *unaligned_io || *extend ||
>>> -         !ext4_should_dioread_nolock(inode) ||
>>> +         !ext4_dio_should_shared_lock(inode) ||
>>>            !ext4_overwrite_io(inode, offset, count))) {
>>>           ext4_iunlock(inode, *iolock);
>>>           *iolock = EXT4_IOLOCK_EXCL;
>>> @@ -485,7 +495,10 @@ static ssize_t ext4_dio_write_iter(struct kiocb 
>>> *iocb, struct iov_iter *from)
>>>           iolock = EXT4_IOLOCK_EXCL;
>>>       }
>>> -    if (iolock == EXT4_IOLOCK_SHARED && 
>>> !ext4_should_dioread_nolock(inode))
>>> +    /*
>>> +     * Check if we should continue with shared iolock
>>> +     */
>>> +    if (iolock == EXT4_IOLOCK_SHARED && 
>>> !ext4_dio_should_shared_lock(inode))
>>>           iolock = EXT4_IOLOCK_EXCL;
>>>       if (iocb->ki_flags & IOCB_NOWAIT) {
>>> -- 
>>> 2.21.0
>>>
>