Re: [PATCH v19 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  1 Jul 2024 15:57:55 +0200 "Jason A. Donenfeld" <Jason@zx2c4.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?