From: Andrew Morton <akpm@linux-foundation.org>
To: Zan Lynx <zlynx@acm.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
linux-kernel@vger.kernel.org, Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH 1/2] msync: support syncing a small part of the file
Date: Wed, 13 Jun 2012 15:08:09 -0700 [thread overview]
Message-ID: <20120613150809.44149ef6.akpm@linux-foundation.org> (raw)
In-Reply-To: <1339624293.2080.24.camel@knife>
On Wed, 13 Jun 2012 15:51:33 -0600
Zan Lynx <zlynx@acm.org> wrote:
> On Wed, 2012-06-13 at 14:26 -0700, Andrew Morton wrote:
> > On Thu, 31 May 2012 22:43:54 +0200
> > Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > > msync does not need to flush changes to the entire file, even with MS_SYNC.
> > > Instead, it can use vfs_fsync_range to only synchronize a part of the file.
> > >
> > > In addition, not all metadata has to be synced; msync is closer to
> > > fdatasync than it is to msync. So, pass 1 to vfs_fsync_range.
> >
> > Would be nice, but if applications were previously assuming that an
> > msync() was syncing the whole file, this patch will secretly and subtly
> > break them. Convince me that this change won't weaken anyone's data
> > integrity ;)
>
> As an interested observer and a programmer who uses msync()...
>
> I never assumed msync() did the whole file.
OK, that's one user accounted for. 3 million to go.
Look, I'm not terribly averse to the change, but it does have this
risk. And it's a nasty risk because anyone who is hit by it simply
will not know that his applcation has lost some of its data integrity.
> It has an address and length
> argument. I always assumed it only looked for dirty pages within that
> range. That is also what the msync() documentation claims.
>
> As for weakening data integrity because of assumptions programmers *may*
> have made, I think this is a bad argument which followed to its logical
> conclusion can only lead to requiring an implicit sync() before and
> after every system call. :-)
No, not at all. The issue is the *removal* of a side-effect upon which
some apps/designers may have been depending. Perhaps unintentionally!
next prev parent reply other threads:[~2012-06-13 22:08 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-05-31 20:43 [PATCH 0/2] msync improvements Paolo Bonzini
2012-05-31 20:43 ` [PATCH 1/2] msync: support syncing a small part of the file Paolo Bonzini
2012-06-13 21:26 ` Andrew Morton
2012-06-13 21:51 ` Zan Lynx
2012-06-13 22:08 ` Andrew Morton [this message]
2012-06-14 8:57 ` Paolo Bonzini
2012-05-31 20:43 ` [PATCH 2/2] msync: start async writeout when MS_ASYNC Paolo Bonzini
2012-06-13 21:29 ` Andrew Morton
2012-06-14 9:02 ` Paolo Bonzini
2012-06-14 10:07 ` Andrew Morton
2012-06-14 10:19 ` Paolo Bonzini
2012-06-14 12:24 ` Paolo Bonzini
2012-06-12 15:38 ` [PATCH 0/2] msync improvements Paolo Bonzini
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