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: Tue, 4 Jun 2024 09:34:48 +0900
From: Byungchul Park <byungchul@...com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Dave Hansen <dave.hansen@...el.com>,
	David Hildenbrand <david@...hat.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 06:01:05PM +0100, Matthew Wilcox wrote:
> 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

At this point, tlb flush needed will be performed in prep_new_page().

>  - application reads through stale TLB entry and sees kernel memory

No worry for this case.

	Byungchul
> 
> Or did that scenario get resolved?