Re: [PATCH v3 0/7] File Sealing & memfd_create()

[David Herrmann]

Hi

On Tue, Jun 17, 2014 at 6:20 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> Can you summarize why holes can't be reliably backed by the zero page?

To answer this, I will quote Hugh from "PATCH v2 1/3":

> We do already use the ZERO_PAGE instead of allocating when it's a
> simple read; and on the face of it, we could extend that to mmap
> once the file is sealed.  But I am rather afraid to do so - for
> many years there was an mmap /dev/zero case which did that, but
> it was an easily forgotten case which caught us out at least
> once, so I'm reluctant to reintroduce it now for sealing.
>
> Anyway, I don't expect you to resolve the issue of sealed holes:
> that's very much my territory, to give you support on.

Holes can be avoided with a simple fallocate(). I don't understand why
I should make SEAL_WRITE do the fallocate for the caller. During the
discussion of memfd_create() I was told to drop the "size" parameter,
because it is redundant. I don't see how this implicit fallocate()
does not fall into the same category?

Thanks
David
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