linux-kernel - Re: [RFC PATCH v3 00/10] Add support for shared PTEs across processes

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Message-ID: <9ede93dc-01cb-4497-a67b-cb03e864e369@oracle.com>
Date: Wed, 2 Oct 2024 17:24:36 -0700
From: Anthony Yznaga <anthony.yznaga@...cle.com>
To: Dave Hansen <dave.hansen@...el.com>, akpm@...ux-foundation.org,
        willy@...radead.org, markhemm@...glemail.com, viro@...iv.linux.org.uk,
        david@...hat.com, khalid@...nel.org
Cc: andreyknvl@...il.com, luto@...nel.org, brauner@...nel.org, arnd@...db.de,
        ebiederm@...ssion.com, catalin.marinas@....com,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mhiramat@...nel.org, rostedt@...dmis.org,
        vasily.averin@...ux.dev, xhao@...ux.alibaba.com, pcc@...gle.com,
        neilb@...e.de, maz@...nel.org, David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC PATCH v3 00/10] Add support for shared PTEs across processes


On 10/2/24 4:11 PM, Dave Hansen wrote:
> About TLB flushing...
>
> The quick and dirty thing to do is just flush_tlb_all() after you remove
> the PTE from the host mm.  That will surely work everywhere and it's as
> dirt simple as you get.  Honestly, it might even be cheaper than the
> alternative.

I think this a good place to start from. My concern is that unnecessary 
flushes will potentially impact unrelated loads. Performance testing as 
things progress can help determine if a more involved approach is needed.

>
> Also, I don't think PCIDs actually complicate the problem at all.  We
> basically do remote mm TLB flushes using two mechanisms:
>
> 	1. If the mm is loaded, use INVLPG and friends to zap the TLB
> 	2. Bump mm->context.tlb_gen so that the next time it _gets_
> 	   loaded, the TLB is flushed.
>
> flush_tlb_func() really only cares about #1 since if the mm isn't
> loaded, it'll get flushed anyway at the next context switch.
>
> The alternatives I can think of:
>
> Make flush_tlb_mm_range(host_mm) work somehow.  You'd need to somehow
> keep mm_cpumask(host_mm) up to date and also make do something to
> flush_tlb_func() to tell it that 'loaded_mm' isn't relevant and it
> should flush regardless.
>
> The other way is to use the msharefs's inode ->i_mmap to find all the
> VMAs mapping the file, and find all *their* mm's:
>
> 	for each vma in inode->i_mmap
> 		mm = vma->vm_mm
> 		flush_tlb_mm_range(<vma range here>)
>
> But that might be even worse than flush_tlb_all() because it might end
> up sending more than one IPI per CPU.
>
> You can fix _that_ by keeping a single cpumask that you build up:
>
> 	mask = 0
> 	for each vma in inode->i_mmap
> 		mm = vma->vm_mm
> 		mask |= mm_cpumask(mm)
>
> 	flush_tlb_multi(mask, info);
>
> Unfortunately, 'info->mm' needs to be more than one mm, so you probably
> still need a new flush_tlb_func() flush type to tell it to ignore
> 'info->mm' and flush anyway.

What about something like arch_tlbbatch_flush() which sets info->mm to 
NULL and uses a batch cpumask? That seems perfect though there would be 
a bit more work needed to ensure things work on other architectures. 
flush_tlb_all() it is then, for now. :-)

>
> After all that, I kinda like flush_tlb_all(). ;)