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

[David Herrmann]

Hi

On Tue, Jun 17, 2014 at 6:41 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Tue, Jun 17, 2014 at 9:36 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
>> 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?
>>
>
> 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?

I assumed Hugh was talking about read(). So no, this is not about
memory-reads on mmap()ed regions.

Looking at shmem_file_read_iter() I can see a ZERO_PAGE(0) call in
case shmem_getpage_gfp(SGP_READ) tells us there's a hole. I cannot see
anything like that in the mmap_region() and shmem_fault() paths.

Thanks
David
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