Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

[David Hildenbrand]

Am 10.05.24 um 08:51 schrieb Byungchul Park:
> Hi everyone,
>
> While I'm working with a tiered memory system e.g. CXL memory, I have
> been facing migration overhead esp. tlb shootdown on promotion or
> demotion between different tiers.  Yeah..  most tlb shootdowns on
> migration through hinting fault can be avoided thanks to Huang Ying's
> work, commit 4d4b6d66db ("mm,unmap: avoid flushing tlb in batch if PTE
> is inaccessible").  See the following link for more information:
>
> https://lore.kernel.org/lkml/20231115025755.GA29979@xxxxxxxxxxxxxxxxxxx/
>
> However, it's only for migration through hinting fault.  I thought it'd
> be much better if we have a general mechanism to reduce all the tlb
> numbers that we can apply to any unmap code, that we normally believe
> tlb flush should be followed.
>
> I'm suggesting a new mechanism, LUF(Lazy Unmap Flush), defers tlb flush
> until folios that have been unmapped and freed, eventually get allocated
> again.  It's safe for folios that had been mapped read-only and were
> unmapped, since the contents of the folios don't change while staying in
> pcp or buddy so we can still read the data through the stale tlb entries.
>
> tlb flush can be defered when folios get unmapped as long as it
> guarantees to perform tlb flush needed, before the folios actually
> become used, of course, only if all the corresponding ptes don't have
> write permission.  Otherwise, the system will get messed up.
>
> To achieve that:
>
>     1. For the folios that map only to non-writable tlb entries, prevent
>        tlb flush during unmapping but perform it just before the folios
>        actually become used, out of buddy or pcp.

Trying to understand the impact: Effectively, a CPU could still read data from a 
page that has already been freed, until that page gets reallocated again.

The important part I can see is

1) PCP/buddy must not change page content (e.g., poison, init_on_free), 
otherwise an app might read wrong content.

2) If we mess up the flush-before-realloc, an app might observe data written by 
whoever allocated the page.

3) We must reliably detect+handle any read-only PTEs for which we didn't flush 
the TLB yet, otherwise an app could see its memory writes getting lost. I recall 
that at least uffd-wp might defer TLB flushes (see comment in do_wp_page()). Not 
sure about other pte_wrprotect() callers that flush the TLB after processing 
multiple page tables, whereby rmap code might succeed in unmapping a page before 
the TLB flush happened.

Any other possible issues you stumbled over that are worth mentioning?

--
Thanks,

David / dhildenb