linux-kernel - Re: [PATCH v2 7/8] execmem: add support for cache of large ROX pages

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Message-ID: <7c3720ff-b763-44b0-9b57-a2fbff3c7f56@csgroup.eu>
Date: Fri, 6 Sep 2024 11:37:34 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Mike Rapoport <rppt@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>
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Subject: Re: [PATCH v2 7/8] execmem: add support for cache of large ROX pages

Le 26/08/2024 à 08:55, Mike Rapoport a écrit :
> From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> 
> Using large pages to map text areas reduces iTLB pressure and improves
> performance.
> 
> Extend execmem_alloc() with an ability to use PMD_SIZE'ed pages with ROX
> permissions as a cache for smaller allocations.

Why only PMD_SIZE ?

On power 8xx, PMD_SIZE is 4M and the 8xx doesn't have such a page size. 
When you call vmalloc() with VM_ALLOW_HUGE_VMAP you get 16k pages or 
512k pages depending on the size you ask for, see function 
arch_vmap_pte_supported_shift()

> 
> To populate the cache, a writable large page is allocated from vmalloc with
> VM_ALLOW_HUGE_VMAP, filled with invalid instructions and then remapped as
> ROX.
> 
> Portions of that large page are handed out to execmem_alloc() callers
> without any changes to the permissions.
> 
> When the memory is freed with execmem_free() it is invalidated again so
> that it won't contain stale instructions.
> 
> The cache is enabled when an architecture sets EXECMEM_ROX_CACHE flag in
> definition of an execmem_range.
> 

Christophe