Re: [PATCH 1/2] msync: support syncing a small part of the file

[Andrew Morton]

On Wed, 13 Jun 2012 15:51:33 -0600
Zan Lynx <zlynx@xxxxxxx> 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@xxxxxxxxxx> 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!

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