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Message-Id: <20240702004254.3ab2db4a98cb7fdd245407cb@linux-foundation.org>
Date: Tue, 2 Jul 2024 00:42:54 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
tglx@...utronix.de, linux-crypto@...r.kernel.org,
linux-api@...r.kernel.org, x86@...nel.org, Greg Kroah-Hartman
<gregkh@...uxfoundation.org>, Adhemerval Zanella Netto
<adhemerval.zanella@...aro.org>, Carlos O'Donell <carlos@...hat.com>,
Florian Weimer <fweimer@...hat.com>, Arnd Bergmann <arnd@...db.de>, Jann
Horn <jannh@...gle.com>, Christian Brauner <brauner@...nel.org>, David
Hildenbrand <dhildenb@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH v19 1/5] mm: add VM_DROPPABLE for designating always
lazily freeable mappings
On Mon, 1 Jul 2024 15:57:55 +0200 "Jason A. Donenfeld" <Jason@...c4.com> wrote:
> The vDSO getrandom() implementation works with a buffer allocated with a
> new system call that has certain requirements:
>
> - It shouldn't be written to core dumps.
> * Easy: VM_DONTDUMP.
> - It should be zeroed on fork.
> * Easy: VM_WIPEONFORK.
>
> - It shouldn't be written to swap.
> * Uh-oh: mlock is rlimited.
> * Uh-oh: mlock isn't inherited by forks.
>
> It turns out that the vDSO getrandom() function has three really nice
> characteristics that we can exploit to solve this problem:
>
> 1) Due to being wiped during fork(), the vDSO code is already robust to
> having the contents of the pages it reads zeroed out midway through
> the function's execution.
>
> 2) In the absolute worst case of whatever contingency we're coding for,
> we have the option to fallback to the getrandom() syscall, and
> everything is fine.
>
> 3) The buffers the function uses are only ever useful for a maximum of
> 60 seconds -- a sort of cache, rather than a long term allocation.
>
> These characteristics mean that we can introduce VM_DROPPABLE, which
> has the following semantics:
>
> a) It never is written out to swap.
> b) Under memory pressure, mm can just drop the pages (so that they're
> zero when read back again).
> c) It is inherited by fork.
> d) It doesn't count against the mlock budget, since nothing is locked.
>
> This is fairly simple to implement, with the one snag that we have to
> use 64-bit VM_* flags, but this shouldn't be a problem, since the only
> consumers will probably be 64-bit anyway.
>
> This way, allocations used by vDSO getrandom() can use:
>
> VM_DROPPABLE | VM_DONTDUMP | VM_WIPEONFORK | VM_NORESERVE
>
> And there will be no problem with using memory when not in use, not
> wiping on fork(), coredumps, or writing out to swap.
The patch is impressively comment-free. It is a little harsh to make
readers go poking around in the git history to figure out what
VM_DROPPABLE is, and why it exists.
Seems hard to test that this mode is working correctly. Can you think
of a way for userspace to check this? And if so, add it to selftests?
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