Re: ext4 DIO read performance issue on SSD

[Jiaying Zhang <jiayingz@google.com>]

Thinking about this again, I think there is another benefit to always
allocate uninitialized blocks first in all kinds of write, including
size-extending write.

Currently, we avoid exposing stale data during size-extending
write by adding the inode to the orphan list before changing the
file size. This helps us with journaled ext4 because the journal
guarantees that the on-disk orphan list will be updated first before
data is written to disk and i_size is updated. But with non-journal ext4,
we don't update orphan list during size-extending write because
without a journal, there is no guarantee on the ordering of writes to disk.
So if the system crashes after the on-disk i_size is updated but before
data hits to disk (this is rare but can happen during fsync), we may
end up exposing stale data with non-journal ext4.

I think allocating uninitialized extend first during such size-extending
write and then converting the extend into initialized during end_io
time can help close this security hole with non-journal ext4.

Any thoughts on this?

Jiaying
On Thu, Oct 15, 2009 at 10:27 AM, Mingming <cmm@us.ibm.com> wrote:
> On Wed, 2009-10-14 at 14:42 -0700, Jiaying Zhang wrote:
>> On Wed, Oct 14, 2009 at 1:57 PM, Mingming <cmm@us.ibm.com> wrote:
>> > On Wed, 2009-10-14 at 12:48 -0700, Jiaying Zhang wrote:
>> >> On Wed, Oct 14, 2009 at 11:48 AM, Mingming <cmm@us.ibm.com> wrote:
>> >> > On Fri, 2009-10-09 at 16:34 -0700, Jiaying Zhang wrote:
>> >> >> Hello,
>> >> >>
>> >> >> Recently, we are evaluating the ext4 performance on a high speed SSD.
>> >> >> One problem we found is that ext4 performance doesn't scale well with
>> >> >> multiple threads or multiple AIOs reading a single file with O_DIRECT.
>> >> >> E.g., with 4k block size, multiple-thread DIO AIO random read on ext4
>> >> >> can lose up to 50% throughput compared to the results we get via RAW IO.
>> >> >>
>> >> >> After some initial analysis, we think the ext4 performance problem is caused
>> >> >> by the use of i_mutex lock during DIO read. I.e., during DIO read, we grab
>> >> >> the i_mutex lock in __blockdev_direct_IO because ext4 uses the default
>> >> >> DIO_LOCKING from the generic fs code. I did a quick test by calling
>> >> >> blockdev_direct_IO_no_locking() in ext4_direct_IO() and I saw ext4 DIO read
>> >> >> got 99% performance as raw IO.
>> >> >>
>> >> >
>> >> > This is very interesting...and impressive number.
>> >> >
>> >> > I tried to change ext4 to call blockdev_direct_IO_no_locking() directly,
>> >> > but then realize that we can't do this all the time, as ext4 support
>> >> > ext3 non-extent based files, and uninitialized extent is not support on
>> >> > ext3 format file.
>> >> >
>> >> >> As we understand, the reason why we want to take i_mutex lock during DIO
>> >> >> read is to prevent it from accessing stale data that may be exposed by a
>> >> >> simultaneous write. We saw that Mingming Cao has implemented a patch set
>> >> >> with which when a get_block request comes from direct write, ext4 only
>> >> >> allocates or splits an uninitialized extent. That uninitialized extent
>> >> >> will be marked as initialized at the end_io callback.
>> >> >
>> >> > Though I need to clarify that with all the patches in mainline, we only
>> >> > treat new allocated blocks form direct io write to holes, not to writes
>> >> > to the end of file. I actually have proposed to treat the write to the
>> >> > end of file also as unintialized extents, but there is some concerns
>> >> > that this getting tricky with updating inode size when it is async IO
>> >> > direct IO. So it didn't getting done yet.
>> >> >
>> >> >>  We are wondering
>> >> >> whether we can extend this idea to buffer write as well. I.e., we always
>> >> >> allocate an uninitialized extent first during any write and convert it
>> >> >> as initialized at the time of end_io callback. This will eliminate the need
>> >> >> to hold i_mutex lock during direct read because a DIO read should never get
>> >> >> a block marked initialized before the block has been written with new data.
>> >> >>
>> >> >
>> >> > Oh I don't think so. For buffered IO, the data is being copied to
>> >> > buffer, direct IO read would first flush what's in page cache to disk,
>> >>
>> >> Hmm, do you mean the filemap_write_and_wait_range() in
>> >> __blockdev_direct_IO?
>> >
>> > yes, that's the one to flush the page cache before direct read.
>> >
>> I don't think that function is called with DIO_NO_LOCKING.
>
> Oh, I mean the filemap_write_and_wait_range() in generic_file_aio_read()
>
>> Also, if we no longer hold i_mutex lock during dio read, I think
>> there is a time window that a buffer write can allocate an
>> initialize block after dio read flushes page cache but
>> before it calls get_block. Then that dio read can get that
>> initialized block with stale data.
>>
>
> ah, I think it over, the key is prevent get_block() expose initialized
> extent to direct read. concurrent buffered write to hole could result in
> get_block() allocate blocks before direct IO read. That could be
> addressed in a similar way we did for async direct IO write to hole...
>> Jiaying
>>
>> >> Or do we flush page cache after calling
>> >> get_block in dio read?
>> >>
>> >> Jiaying
>> >>
>> >> > then read from disk. So if there is concurrent buffered write and direct
>> >> > read, removing the i_mutex locks from the direct IO path should still
>> >> > gurantee the right order, without having to treat buffered allocation
>> >> > with uninitialized extent/end_io.
>> >> >
>> >> > The i_mutex lock, from my understanding, is there to protect direct IO
>> >> > write to hole and concurrent direct IO read, we should able to remove
>> >> > this lock for extent based ext4 file.
>> >> >
>> >>
>> >>
>> >> >> We haven't implemented anything yet because we want to ask here first to
>> >> >> see whether this proposal makes sense to you.
>> >> >>
>> >> >
>> >> > It does make sense to me.
>> >> >
>> >> > Mingming
>> >> >> Regards,
>> >> >>
>> >> >> Jiaying
>> >> >> --
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>> >> >
>> >> >
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>> >
>> >
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