Re: [PATCH v23 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings

[Miaohe Lin <linmiaohe@huawei.com>]

On 2024/7/12 9:40, 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 shouldn't reserve actual memory, but it also shouldn't crash when
>   page faulting in memory if none is available
>   * Uh-oh: VM_NORESERVE means segfaults.
> 
> 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.
> e) If there's not enough memory to service a page fault, it's not fatal,
>    and no signal is sent.
> 
> This way, allocations used by vDSO getrandom() can use:
> 
>     VM_DROPPABLE | VM_DONTDUMP | VM_WIPEONFORK | VM_NORESERVE
> 
> And there will be no problem with OOMing, crashing on overcommitment,
> 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.
> 
> Note that this involves removing the MADV_FREE special case from
> sort_folio(), which according to Yu Zhao is unnecessary and will simply
> result in an extra call to shrink_folio_list() in the worst case. The
> chunk removed reenables the swapbacked flag, which we don't want for
> VM_DROPPABLE, and we can't conditionalize it here because there isn't a
> vma reference available.
> 
> Finally, the provided self test ensures that this is working as desired.
> 
> Cc: linux-mm@kvack.org
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
...

> diff --git a/mm/memory.c b/mm/memory.c
> index d10e616d7389..18fe893ce96d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5690,6 +5690,10 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  
>  	lru_gen_exit_fault();
>  
> +	/* If the mapping is droppable, then errors due to OOM aren't fatal. */
> +	if (vma->vm_flags & VM_DROPPABLE)
> +		ret &= ~VM_FAULT_OOM;
> +
I'm sorry for jumping in here. I am confused about the code in handle_mm_fault(). Since VM_FAULT_OOM is simply
dropped, page fault will be re-triggered soon? If so, when oom is disabled or fails to move forward, page fault
will re-trigger again and again as no memory is available? I might be miss something.

Thanks.
.