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Date:   Wed, 18 Sep 2019 08:58:17 +0800
From:   Joseph Qi <joseph.qi@...ux.alibaba.com>
To:     Ritesh Harjani <riteshh@...ux.ibm.com>, jack@...e.cz,
        tytso@....edu, linux-ext4@...r.kernel.org
Cc:     david@...morbit.com, hch@...radead.org, adilger@...ger.ca,
        mbobrowski@...browski.org, rgoldwyn@...e.de
Subject: Re: [RFC 0/2] ext4: Improve locking sequence in DIO write path



On 19/9/17 18:32, Ritesh Harjani wrote:
> Hello,
> 
> This patch series is based on the upstream discussion with Jan
> & Joseph @ [1].
> It is based on top of Matthew's v3 ext4 iomap patch series [2]
> 
> Patch-1: Adds the ext4_ilock/unlock APIs and also replaces all
> inode_lock/unlock instances from fs/ext4/*
> 
> For now I already accounted for trylock/lock issue symantics
> (which was discussed here [3]) in the same patch,
> since the this whole patch was around inode_lock/unlock API,
> so I thought it will be best to address that issue in the same patch. 
> However, kindly let me know if otherwise.
> 
> Patch-2: Commit msg of this patch describes in detail about
> what it is doing.
> In brief - we try to first take the shared lock (instead of exclusive
> lock), unless it is a unaligned_io or extend_io. Then in
> ext4_dio_write_checks(), if we start with shared lock, we see
> if we can really continue with shared lock or not. If not, then
> we release the shared lock then acquire exclusive lock
> and restart ext4_dio_write_checks().
> 
> 
> Tested against few xfstests (with dioread_nolock mount option),
> those ran fine (ext4 & generic).
> 
> I tried testing performance numbers on my VM (since I could not get
> hold of any real h/w based test device). I could test the fact
> that earlier we were trying to do downgrade_write() lock, but with
> this patch, that path is now avoided for fio test case
> (as reported by Joseph in [4]).
> But for the actual results, I am not sure if VM machine testing could
> really give the reliable perf numbers which we want to take a look at.
> Though I do observe some form of perf improvements, but I could not
> get any reliable numbers (not even with the same list of with/without
> patches with which Joseph posted his numbers [1]).
> 
> 
> @Joseph,
> Would it be possible for you to give your test case a run with this
> patches? That will be really helpful.
> 

Sure, will post the result ASAP.

Thanks,
Joseph

> Branch for this is hosted at below tree.
> 
> https://github.com/riteshharjani/linux/tree/ext4-ilock-RFC
> 
> [1]: https://lore.kernel.org/linux-ext4/20190910215720.GA7561@quack2.suse.cz/
> [2]: https://lwn.net/Articles/799184/
> [3]: https://lore.kernel.org/linux-fsdevel/20190911103117.E32C34C044@d06av22.portsmouth.uk.ibm.com/
> [4]: https://lore.kernel.org/linux-ext4/1566871552-60946-4-git-send-email-joseph.qi@linux.alibaba.com/
> 
> 
> Ritesh Harjani (2):
>   ext4: Add ext4_ilock & ext4_iunlock API
>   ext4: Improve DIO writes locking sequence
> 
>  fs/ext4/ext4.h    |  33 ++++++
>  fs/ext4/extents.c |  16 +--
>  fs/ext4/file.c    | 253 ++++++++++++++++++++++++++++++++--------------
>  fs/ext4/inode.c   |   4 +-
>  fs/ext4/ioctl.c   |  16 +--
>  fs/ext4/super.c   |  12 +--
>  fs/ext4/xattr.c   |  16 +--
>  7 files changed, 244 insertions(+), 106 deletions(-)
> 

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