linux-kernel - Re: [PATCH v3 0/7] File Sealing & memfd

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Message-ID: <CANq1E4QinvVA-O=dX4N819jVK3xSihJcpz9juFj+A3qv0MXODg@mail.gmail.com>
Date:	Tue, 17 Jun 2014 12:10:20 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Florian Weimer <fweimer@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Ryan Lortie <desrt@...rt.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Greg Kroah-Hartman <greg@...ah.com>,
	John Stultz <john.stultz@...aro.org>,
	Lennart Poettering <lennart@...ttering.net>,
	Daniel Mack <zonque@...il.com>, Kay Sievers <kay@...y.org>,
	Hugh Dickins <hughd@...gle.com>,
	Tony Battersby <tonyb@...ernetics.com>
Subject: Re: [PATCH v3 0/7] File Sealing & memfd_create()

Hi

On Tue, Jun 17, 2014 at 12:04 PM, Florian Weimer <fweimer@...hat.com> wrote:
> On 06/17/2014 12:01 PM, David Herrmann wrote:
>
>>> I don't think this is what potential users expect because mlock requires
>>> capabilities which are not available to them.
>>>
>>> A couple of weeks ago, sealing was to be applied to anonymous shared
>>> memory.
>>> Has this changed?  Why should *reading* it trigger OOM?
>>
>> The file might have holes, therefore, you'd have to allocate backing
>> pages. This might hit a soft-limit and fail. To avoid this, use
>> fallocate() to allocate pages prior to mmap()
>
> This does not work because the consuming side does not know how the
> descriptor was set up if sealing does not imply that.

The consuming side has to very seals via F_GET_SEALS. After that, it
shall do a simple fallocate() on the whole file if it wants to go sure
that all pages are allocated. Why shouldn't that be possible? Please
elaborate.

Thanks
David
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