Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

[Nadav Amit]

> On Dec 23, 2020, at 7:09 PM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
> 
>> On Dec 23, 2020, at 6:00 PM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
>> 
>> On Wed, Dec 23, 2020 at 05:21:43PM -0800, Andy Lutomirski wrote:
>>> I don’t love this as a long term fix. AFAICT we can have mm_tlb_flush_pending set for quite a while — mprotect seems like it can wait in IO while splitting a huge page, for example. That gives us a window in which every write fault turns into a TLB flush.
>> 
>> mprotect can't run concurrently with a page fault in the first place.
>> 
>> One other near zero cost improvement easy to add if this would be "if
>> (vma->vm_flags & (VM_SOFTDIRTY|VM_UFFD_WP))" and it could be made
>> conditional to the two config options too.
>> 
>> Still I don't mind doing it in some other way, uffd-wp has much easier
>> time doing it in another way in fact.
>> 
>> Whatever performs better is fine, but queuing up pending invalidate
>> ranges don't look very attractive since it'd be a fixed cost that we'd
>> always have to pay even when there's no fault (and there can't be any
>> fault at least for mprotect).
> 
> I think there are other cases in which Andy’s concern is relevant
> (MADV_PAGEOUT).
> 
> Perhaps holding some small bitmap based on part of the deferred flushed
> pages (e.g., bits 12-17 of the address or some other kind of a single
> hash-function bloom-filter) would be more performant to avoid (most)
> unnecessary TLB flushes. It will be cleared before a TLB flush and set while
> holding the PTL.
> 
> Checking if a flush is needed, under the PTL, would require a single memory
> access (although potentially cache miss). It will however require one atomic
> operation for each page-table whose PTEs’ flushes are deferred - in contrast
> to the current scheme which requires two atomic operations for the *entire*
> operation.

Just to finish my thought - clearing the bitmap is the tricky part,
which I still did not figure out.