On Tue, Jun 17, 2014 at 9:36 AM, David Herrmann [off-list ref] wrote:

Hi

On Tue, Jun 17, 2014 at 6:20 PM, Andy Lutomirski [off-list ref] wrote:

quoted

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":

quoted

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?

I'm really confused now.

If I SEAL_WRITE a file, and then I mmap it PROT_READ, and then I read
it, is that a "simple read"?  If so, doesn't that mean that there's no
problem?

--Andy

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