linux-kernel - Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings

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Message-ID: <378f23cb-362e-413a-b221-09a5352e79f2@redhat.com>
Date: Wed, 10 Jul 2024 05:27:34 +0200
From: David Hildenbrand <david@...hat.com>
To: "Jason A. Donenfeld" <Jason@...c4.com>, linux-kernel@...r.kernel.org,
 patches@...ts.linux.dev, tglx@...utronix.de
Cc: linux-crypto@...r.kernel.org, linux-api@...r.kernel.org, x86@...nel.org,
 Linus Torvalds <torvalds@...ux-foundation.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 v22 1/4] mm: add MAP_DROPPABLE for designating always
 lazily freeable mappings

On 09.07.24 15:05, Jason A. Donenfeld 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.
> 
> In order to let vDSO getrandom() use this, expose these via mmap(2) as
> MAP_DROPPABLE.
> 
> Finally, the provided self test ensures that this is working as desired.

Acked-by: David Hildenbrand <david@...hat.com>

I'll try to think of some corner cases we might be missing.

As raised, I think we could do better at naming, such as "MAP_FREEABLE" 
to match MADV_FREE, MAP_VOLATILE, ... but if nobody else care, I shall 
not care :)

-- 
Cheers,

David / dhildenb