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

[David Hildenbrand <david@redhat.com>]

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@redhat.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