linux-kernel - Re: [PATCH 3/8] x86/mm/pat: Restore large pages after fragmentation

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Message-ID: <Z4FyiALKwX-AIWtF@bombadil.infradead.org>
Date: Fri, 10 Jan 2025 11:18:32 -0800
From: Luis Chamberlain <mcgrof@...nel.org>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Mike Rapoport <rppt@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Anton Ivanov <anton.ivanov@...bridgegreys.com>,
	Borislav Petkov <bp@...en8.de>,
	Brendan Higgins <brendan.higgins@...ux.dev>,
	Daniel Gomez <da.gomez@...sung.com>,
	Daniel Thompson <danielt@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	David Gow <davidgow@...gle.com>,
	Douglas Anderson <dianders@...omium.org>,
	Ingo Molnar <mingo@...hat.com>,
	Jason Wessel <jason.wessel@...driver.com>,
	Jiri Kosina <jikos@...nel.org>,
	Joe Lawrence <joe.lawrence@...hat.com>,
	Johannes Berg <johannes@...solutions.net>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mark Rutland <mark.rutland@....com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Miroslav Benes <mbenes@...e.cz>, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Petr Mladek <pmladek@...e.com>, Petr Pavlu <petr.pavlu@...e.com>,
	Rae Moar <rmoar@...gle.com>, Richard Weinberger <richard@....at>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Shuah Khan <shuah@...nel.org>, Song Liu <song@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	kgdb-bugreport@...ts.sourceforge.net, kunit-dev@...glegroups.com,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-mm@...ck.org, linux-modules@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, linux-um@...ts.infradead.org,
	live-patching@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 3/8] x86/mm/pat: Restore large pages after fragmentation

On Fri, Jan 10, 2025 at 12:36:59PM +0200, Kirill A. Shutemov wrote:
> On Fri, Dec 27, 2024 at 09:28:20AM +0200, Mike Rapoport wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > 
> > Change of attributes of the pages may lead to fragmentation of direct
> > mapping over time and performance degradation as result.
> > 
> > With current code it's one way road: kernel tries to avoid splitting
> > large pages, but it doesn't restore them back even if page attributes
> > got compatible again.
> > 
> > Any change to the mapping may potentially allow to restore large page.
> > 
> > Hook up into cpa_flush() path to check if there's any pages to be
> > recovered in PUD_SIZE range around pages we've just touched.
> > 
> > CPUs don't like[1] to have to have TLB entries of different size for the
> > same memory, but looks like it's okay as long as these entries have
> > matching attributes[2]. Therefore it's critical to flush TLB before any
> > following changes to the mapping.
> > 
> > Note that we already allow for multiple TLB entries of different sizes
> > for the same memory now in split_large_page() path. It's not a new
> > situation.
> > 
> > set_memory_4k() provides a way to use 4k pages on purpose. Kernel must
> > not remap such pages as large. Re-use one of software PTE bits to
> > indicate such pages.
> > 
> > [1] See Erratum 383 of AMD Family 10h Processors
> > [2] https://lore.kernel.org/linux-mm/1da1b025-cabc-6f04-bde5-e50830d1ecf0@amd.com/
> > 
> > [rppt@...nel.org:
> >  * s/restore/collapse/
> >  * update formatting per peterz
> >  * use 'struct ptdesc' instead of 'struct page' for list of page tables to
> >    be freed
> >  * try to collapse PMD first and if it succeeds move on to PUD as peterz
> >    suggested
> >  * flush TLB twice: for changes done in the original CPA call and after
> >    collapsing of large pages
> > ]
> > 
> > Link: https://lore.kernel.org/all/20200416213229.19174-1-kirill.shutemov@linux.intel.com
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > Co-developed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> 
> When I originally attempted this, the patch was dropped because of
> performance regressions. Was it addressed somehow?
> 

Also, the statement:

"Change of attributes of the pages may lead to fragmentation of
direct mapping over time and performance degradation as result.  "

Seems to contradict the findings reported at LSFMM by Mike before
that direct map fragmentation does not incur performance penalty,
so I don't see a point in contradicing those findings and confusing
people further.

  Luis