linux-kernel - Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering tlb flush when folios get unmapped

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Date: Mon, 3 Jun 2024 18:01:05 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: David Hildenbrand <david@...hat.com>, Byungchul Park <byungchul@...com>,
	Byungchul Park <lkml.byungchul.park@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kernel_team@...ynix.com, akpm@...ux-foundation.org,
	ying.huang@...el.com, vernhao@...cent.com,
	mgorman@...hsingularity.net, hughd@...gle.com, peterz@...radead.org,
	luto@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, rjgolo@...il.com
Subject: Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering
 tlb flush when folios get unmapped

On Mon, Jun 03, 2024 at 09:37:46AM -0700, Dave Hansen wrote:
> Yeah, we'd need some equivalent of a PTE marker, but for the page cache.
>  Presumably some xa_value() that means a reader has to go do a
> luf_flush() before going any farther.

I can allocate one for that.  We've got something like 1000 currently
unused values which can't be mistaken for anything else.

> That would actually have a chance at fixing two issues:  One where a new
> page cache insertion is attempted.  The other where someone goes to look
> in the page cache and takes some action _because_ it is empty (I think
> NFS is doing some of this for file locks).
> 
> LUF is also pretty fundamentally built on the idea that files can't
> change without LUF being aware.  That model seems to work decently for
> normal old filesystems on normal old local block devices.  I'm worried
> about NFS, and I don't know how seriously folks take FUSE, but it
> obviously can't work well for FUSE.

I'm more concerned with:

 - page goes back to buddy
 - page is allocated to slab
 - application reads through stale TLB entry and sees kernel memory

Or did that scenario get resolved?