From: bfields@fieldses.org (J. Bruce Fields)
To: Jeff Layton <jlayton@redhat.com>
Cc: 焦晓冬 <milestonejxd@gmail.com>,
R.E.Wolff@bitwizard.nl, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: POSIX violation by writeback error
Date: Tue, 4 Sep 2018 12:12:03 -0400 [thread overview]
Message-ID: <20180904161203.GD17478@fieldses.org> (raw)
In-Reply-To: <cd137e88c9e882200c08c7336aa7b5a1c84a7ba3.camel@redhat.com>
On Tue, Sep 04, 2018 at 11:44:20AM -0400, Jeff Layton wrote:
> On Tue, 2018-09-04 at 22:56 +0800, 焦晓冬 wrote:
> > A practical and concrete example may be,
> > A disk cleaner program that first searches for garbage files that won't be used
> > anymore and save the list in a file (open()-write()-close()) and wait for the
> > user to confirm the list of files to be removed. A writeback error occurs
> > and the related page/inode/address_space gets evicted while the user is
> > taking a long thought about it. Finally, the user hits enter and the
> > cleaner begin
> > to open() read() the list again. But what gets removed is the old list
> > of files that
> > was generated several months ago...
> >
> > Another example may be,
> > An email editor and a busy mail sender. A well written mail to my boss is
> > composed by this email editor and is saved in a file (open()-write()-close()).
> > The mail sender gets notified with the path of the mail file to queue it and
> > send it later. A writeback error occurs and the related
> > page/inode/address_space gets evicted while the mail is still waiting in the
> > queue of the mail sender. Finally, the mail file is open() read() by the sender,
> > but what is sent is the mail to my girlfriend that was composed yesterday...
> >
> > In both cases, the files are not meant to be persisted onto the disk.
> > So, fsync()
> > is not likely to be called.
> >
>
> So at what point are you going to give up on keeping the data? The
> fundamental problem here is an open-ended commitment. We (justifiably)
> avoid those in kernel development because it might leave the system
> without a way out of a resource crunch.
Well, I think the point was that in the above examples you'd prefer that
the read just fail--no need to keep the data. A bit marking the file
(or even the entire filesystem) unreadable would satisfy posix, I guess.
Whether that's practical, I don't know.
> > - If the following read() could be served by a page in memory, just returns the
> > data. If the following read() could not be served by a page in memory and the
> > inode/address_space has a writeback error mark, returns EIO.
> > If there is a writeback error on the file, and the request data could
> > not be served
> > by a page in memory, it means we are reading a (partically) corrupted
> > (out-of-data)
> > file. Receiving an EIO is expected.
> >
>
> No, an error on read is not expected there. Consider this:
>
> Suppose the backend filesystem (maybe an NFSv3 export) is really r/o,
> but was mounted r/w. An application queues up a bunch of writes that of
> course can't be written back (they get EROFS or something when they're
> flushed back to the server), but that application never calls fsync.
>
> A completely unrelated application is running as a user that can open
> the file for read, but not r/w. It then goes to open and read the file
> and then gets EIO back or maybe even EROFS.
>
> Why should that application (which did zero writes) have any reason to
> think that the error was due to prior writeback failure by a completely
> separate process? Does EROFS make sense when you're attempting to do a
> read anyway?
>
> Moreover, what is that application's remedy in this case? It just wants
> to read the file, but may not be able to even open it for write to issue
> an fsync to "clear" the error. How do we get things moving again so it
> can do what it wants?
>
> I think your suggestion would open the floodgates for local DoS attacks.
Do we really care about processes with write permissions (even only
local client-side write permissions) being able to DoS readers? In
general readers kinda have to trust writers.
--b.
next prev parent reply other threads:[~2018-09-04 16:12 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-04 6:32 POSIX violation by writeback error 焦晓冬
2018-09-04 7:53 ` Rogier Wolff
2018-09-04 8:58 ` 焦晓冬
2018-09-04 9:29 ` Rogier Wolff
2018-09-04 10:45 ` 焦晓冬
2018-09-04 11:09 ` Jeff Layton
2018-09-04 14:56 ` 焦晓冬
2018-09-04 15:44 ` Jeff Layton
2018-09-04 16:12 ` J. Bruce Fields [this message]
2018-09-04 16:23 ` Rogier Wolff
2018-09-04 18:54 ` J. Bruce Fields
2018-09-04 20:18 ` Jeff Layton
2018-09-04 20:35 ` Vito Caputo
2018-09-04 21:02 ` Matthew Wilcox
2018-09-05 0:51 ` Dave Chinner
2018-09-05 8:24 ` 焦晓冬
2018-09-05 10:55 ` Jeff Layton
2018-09-05 12:07 ` Rogier Wolff
2018-09-06 2:57 ` Dave Chinner
2018-09-06 9:17 ` Rogier Wolff
2018-09-24 23:09 ` Alan Cox
2018-09-05 13:53 ` J. Bruce Fields
2018-09-05 7:08 ` Rogier Wolff
2018-09-05 7:39 ` Martin Steigerwald
2018-09-05 8:04 ` Rogier Wolff
2018-09-05 8:37 ` 焦晓冬
2018-09-05 12:07 ` Austin S. Hemmelgarn
2018-09-05 12:46 ` Rogier Wolff
2018-09-05 9:32 ` Martin Steigerwald
2018-09-05 7:37 ` Martin Steigerwald
2018-09-05 11:42 ` Jeff Layton
2018-09-05 8:09 ` 焦晓冬
2018-09-05 13:08 ` Theodore Y. Ts'o
2018-09-24 23:21 ` Alan Cox
2018-09-06 7:28 ` 焦晓冬
[not found] <CAJDTihx2yaR-_-9Ks1PoFcrKNZgUOoLdN-wRTTMV76Jg_dCLrw@mail.gmail.com>
2018-09-04 10:56 ` Jeff Layton
2018-09-24 23:30 ` Alan Cox
2018-09-25 11:15 ` Jeff Layton
2018-09-25 15:46 ` Theodore Y. Ts'o
2018-09-25 16:17 ` Rogier Wolff
2018-09-25 16:39 ` Alan Cox
2018-09-25 16:41 ` Jeff Layton
2018-09-25 22:30 ` Theodore Y. Ts'o
2018-09-26 18:10 ` Alan Cox
2018-09-26 21:49 ` Theodore Y. Ts'o
2018-09-27 22:48 ` Alan Cox
2018-09-27 7:18 ` Rogier Wolff
2018-09-27 12:43 ` Jeff Layton
2018-09-27 14:27 ` Theodore Y. Ts'o
2018-09-25 17:35 ` Adam Borowski
2018-09-25 22:46 ` Theodore Y. Ts'o
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