linux-kernel - Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp

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Date:   Thu, 6 Apr 2023 20:27:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Yair Podemsky <ypodemsk@...hat.com>, linux@...linux.org.uk,
        mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
        hca@...ux.ibm.com, gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
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        alougovs@...hat.com, jannh@...gle.com,
        Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode

On Thu, Apr 06, 2023 at 05:51:52PM +0200, David Hildenbrand wrote:
> On 06.04.23 17:02, Peter Zijlstra wrote:

> > DavidH, what do you thikn about reviving Jann's patches here:
> > 
> >    https://bugs.chromium.org/p/project-zero/issues/detail?id=2365#c1
> > 
> > Those are far more invasive, but afaict they seem to do the right thing.
> > 
> 
> I recall seeing those while discussed on security@...nel.org. What we
> currently have was (IMHO for good reasons) deemed better to fix the issue,
> especially when caring about backports and getting it right.

Yes, and I think that was the right call. However, we can now revisit
without having the pressure of a known defect and backport
considerations.

> The alternative that was discussed in that context IIRC was to simply
> allocate a fresh page table, place the fresh page table into the list
> instead, and simply free the old page table (then using common machinery).
> 
> TBH, I'd wish (and recently raised) that we could just stop wasting memory
> on page tables for THPs that are maybe never going to get PTE-mapped ... and
> eventually just allocate on demand (with some caching?) and handle the
> places where we're OOM and cannot PTE-map a THP in some descend way.
> 
> ... instead of trying to figure out how to deal with these page tables we
> cannot free but have to special-case simply because of GUP-fast.

Not keeping them around sounds good to me, but I'm not *that* familiar
with the THP code, most of that happened after I stopped tracking mm. So
I'm not sure how feasible is it.

But it does look entirely feasible to rework this page-table freeing
along the lines Jann did.