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

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Message-ID: <Zmb-ZZHbeNNjcs68@tiehlicka>
Date: Mon, 10 Jun 2024 15:23:49 +0200
From: Michal Hocko <mhocko@...e.com>
To: Byungchul Park <byungchul@...com>
Cc: Matthew Wilcox <willy@...radead.org>,
	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 Tue 04-06-24 09:34:48, Byungchul Park wrote:
> 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().

But that does mean that an unaware caller would get an additional
overhead of the flushing, right? I think it would be just a matter of
time before somebody can turn that into a side channel attack, not to
mention unexpected latencies introduced.
-- 
Michal Hocko
SUSE Labs