Re: [V2 PATCH] tmpfs: add fallocate support

[Cong Wang]

ä 2011å11æ21æ 18:11, Christoph Hellwig åé:
> On Sun, Nov 20, 2011 at 01:39:12PM -0800, Hugh Dickins wrote:
> > > To be able to safely use mmap(), regarding SIGBUS, on files on the
> > > /dev/shm filesystem. The glibc fallback loop for -ENOSYS on fallocate
> > > is just ugly.
> >
> > The fallback for -EOPNOTSUPP?
>
> Probably for both.  Note that the fallocate man page actually documents
> the errors incorrecly - it documents ENOSYS for filesystems not
> supporting fallocate, and EOPNOTSUPP for not recognizing the mode, but
> we actually return EOPNOTSUPP for either case.  ENOSYS is only returned
> by kernels not implementing fallocate at all.

We need to fix man page of fallocate(2)...

> > Being unfamiliar with glibc, I failed to find the internal_fallocate()
> > that it appears to use when the filesystem doesn't support the call;
> > so I don't know if I would agree with you that it's uglier than doing
> > the same(?) in the kernel.
>
> Last time I looked it basically did a pwrite loop writing zeroes.
> Unfortunately it did far too small I/O sizes and thus actually causes
> some major overhead e.g. on ext3.
>
> > But since the present situation is that tmpfs has one interface to
> > punching holes, madvise(MADV_REMOVE), that IBM were pushing 5 years ago;
> > but ext4 (and others) now a fallocate(FALLOC_FL_PUNCH_HOLE) interface
> > which IBM have been pushing this year: we do want to normalize that
> > situation and make them all behave the same way.
>
> FALLOC_FL_PUNCH_HOLE was added by Josef Bacik, who happens to work for
> Red Hat, but I doubt he was pushing any corporate agenda there, he was
> mostly making btrfs catch up with the 15 year old XFS hole punching
> ioctl.

I sent a patch to util-linux-ng too,

http://thread.gmane.org/gmane.linux.utilities.util-linux-ng/5045

> > And if tmpfs is going to support fallocate(FALLOC_FL_PUNCH_HOLE),
> > looking at Amerigo's much more attractive V2 patch, it would seem
> > to me perverse to permit the deallocation but fail the allocation.
>
> Agreed.

Thanks.
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