linux-kernel - Re: [PATCH v2 6/9] mm: free user PTE page table pages

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Date:   Thu, 2 Sep 2021 14:53:58 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        David Hildenbrand <david@...hat.com>
Cc:     akpm@...ux-foundation.org, tglx@...utronix.de, hannes@...xchg.org,
        mhocko@...nel.org, vdavydov.dev@...il.com,
        kirill.shutemov@...ux.intel.com, mika.penttila@...tfour.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, songmuchun@...edance.com
Subject: Re: [PATCH v2 6/9] mm: free user PTE page table pages



On 2021/9/1 PM11:32, Jason Gunthorpe wrote:
> 
>>> Also, I don't really understand how this scheme works with
>>> get_user_pages_fast.
>>
>> With the RCU change it in #8 it should work just fine, because RCU
>> synchronize has to wait either until all other CPUs have left the RCU read
>> section, or re-enabled interrupts.
> 
> So at this point in the series fast gup is broken, that does mean the
> series presentation really needs to be reworked. The better
> presentation is to add the API changes, with a
> no-functional-difference implementation, push the new API in well
> split patches to all the consumption sites, then change the API to
> have the new semantics.
> 
> RCU and refcount to free the page levels seems like a reasonable
> approach, but I have to say I haven't thought it through fully - are
> all the contexts that have the pte deref safe to do call_rcu?

See Documentation/RCU/rcubarrier.rst:

"Since call_rcu() never blocks, this code can safely be used from within
IRQ context."

So I think call_rcu() can be safely run in any context.

Thinks,
Qi

> 
> Jason
> 
>