Re: [PATCH 01/12] mm/huge_memory: zap deposited page tables after an RCU grace period

From: Jason Gunthorpe <jgg@ziepe.ca>
To: "Liam R. Howlett" <liam@infradead.org>
Cc: Kiryl Shutsemau <kas@kernel.org>,
	"Lorenzo Stoakes (ARM)" <ljs@kernel.org>,
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Subject: Re: [PATCH 01/12] mm/huge_memory: zap deposited page tables after an RCU grace period
Date: Tue, 1 Sep 2026 13:06:16 -0300	[thread overview]
Message-ID: <20260901160616.GD56830@ziepe.ca> (raw)
In-Reply-To: <kasi4mkjkt56wxsrmphm5o2qfg2lwsdxf3tlmzx5tq6t7uez74@kozleoavd6r2>

On Tue, Sep 01, 2026 at 11:54:47AM -0400, Liam R. Howlett wrote:
> On 26/09/01 04:28PM, Kiryl Shutsemau wrote:
> > On Tue, Sep 01, 2026 at 03:41:17PM +0100, Lorenzo Stoakes (ARM) wrote:
> > > On Tue, Sep 01, 2026 at 11:24:08AM -0300, Jason Gunthorpe wrote:
> > > > On Tue, Sep 01, 2026 at 03:12:45PM +0100, Lorenzo Stoakes (ARM) wrote:
> > > >
> > > > > It won't be costly at the time of the calls obviously as its deferred. Maybe
> > > > > increase some time spent in softirq but again is 512x that big of a deal?
> > > > >
> > > > > I'm not sure how you'd both defer the free and somehow utilise mmu_gather here
> > > > > either really, certainly not without it becoming extremely messy.
> > > >
> > > > The less costly version is to thread the page to be freed onto the
> > > > mmu_gather through a linked list in the struct page memory. This is
> > > > super cheap since it is just a singly linked list operation.
> > > >
> > > > Then when the mmu_gather is flushed it does a single call_rcu using
> > > > the rcu head of the struct page of the head of the list. The callback
> > > > clears the entire linked list of pages.
> 
> If you are going through the trouble of doing this on your side with a
> custom rcu callback, it might be better to pack the pointers in a way
> that does not need to pointer chase through a linked list.

Well, I have 8 bytes of memory in a struct page and can't allocate in
these paths, so IDK there is another option..

Jason