linux-kernel - Re: pagecache locking

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <b43e2707-89ec-3afa-8bca-37747ba6c944@gmail.com>
Date:   Sun, 7 Jul 2019 18:05:16 +0300
From:   Boaz Harrosh <openosd@...il.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Dave Chinner <dchinner@...hat.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Josef Bacik <josef@...icpanda.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: pagecache locking

On 06/07/2019 02:31, Dave Chinner wrote:

> 
> As long as the IO ranges to the same file *don't overlap*, it should
> be perfectly safe to take separate range locks (in read or write
> mode) on either side of the mmap_sem as non-overlapping range locks
> can be nested and will not self-deadlock.
> 
> The "recursive lock problem" still arises with DIO and page faults
> inside gup, but it only occurs when the user buffer range overlaps
> the DIO range to the same file. IOWs, the application is trying to
> do something that has an undefined result and is likely to result in
> data corruption. So, in that case I plan to have the gup page faults
> fail and the DIO return -EDEADLOCK to userspace....
> 

This sounds very cool. I now understand. I hope you put all the tools
for this in generic places so it will be easier to salvage.

One thing I will be very curious to see is how you teach lockdep
about the "range locks can be nested" thing. I know its possible,
other places do it, but its something I never understood.

> Cheers,
> Dave.

[ Ha one more question if you have time:

  In one of the mails, and you also mentioned it before, you said about
  the rw_read_lock not being able to scale well on mammoth machines
  over 10ns of cores (maybe you said over 20).
  I wonder why that happens. Is it because of the atomic operations,
  or something in the lock algorithm. In my theoretical understanding,
  as long as there are no write-lock-grabbers, why would the readers
  interfere with each other?
]

Thanks
Boaz