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

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Message-ID: <jut35igb2kstpz24apqdeubv5rvyl3vmp2s43xtivpz54uiedj@wmd2onulv4xw>
Date: Fri, 10 Jan 2025 12:36:59 +0200
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Mike Rapoport <rppt@...nel.org>
Cc: 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>,
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 Johannes Berg <johannes@...solutions.net>,
 	Josh Poimboeuf <jpoimboe@...nel.org>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 	Luis Chamberlain <mcgrof@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 	Masami Hiramatsu <mhiramat@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
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 	live-patching@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 3/8] x86/mm/pat: Restore large pages after fragmentation

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?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov