linux-kernel - Re: [PATCH v2 2/8] x86/mm: Don't do a TLB flush if changing a PTE that isn't marked present

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Date:   Mon, 27 Nov 2023 22:21:07 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "hch@...radead.org" <hch@...radead.org>,
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        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
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        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2 2/8] x86/mm: Don't do a TLB flush if changing a PTE
 that isn't marked present

On Tue, 2023-11-21 at 13:20 -0800, mhkelley58@...il.com wrote:
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -1636,7 +1636,10 @@ static int __change_page_attr(struct cpa_data
> *cpa, int primary)
>                  */
>                 if (pte_val(old_pte) != pte_val(new_pte)) {
>                         set_pte_atomic(kpte, new_pte);
> -                       cpa->flags |= CPA_FLUSHTLB;
> +
> +                       /* If old_pte isn't present, it's not in the
> TLB */
> +                       if (pte_present(old_pte))
> +                               cpa->flags |= CPA_FLUSHTLB;
>                 }
>                 cpa->numpages = 1;
>                 return 0;
> 

Makes sense to me. The PMD case can be handled similarly in
__should_split_large_page().

I also think it should be more robust in regards to the cache flushing
changes.

If callers did:
set_memory_np()
set_memory_uc()
set_memory_p()

Then the cache flush would be missed. I don't think anyone is, but we
shouldn't introduce hidden things like that. Maybe fix it like this:

diff --git a/arch/x86/mm/pat/set_memory.c
b/arch/x86/mm/pat/set_memory.c
index f519e5ca543b..28ff53a4447a 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -1856,11 +1856,6 @@ static int change_page_attr_set_clr(unsigned
long *addr, int numpages,
 
        ret = __change_page_attr_set_clr(&cpa, 1);
 
-       /*
-        * Check whether we really changed something:
-        */
-       if (!(cpa.flags & CPA_FLUSHTLB))
-               goto out;
 
        /*
         * No need to flush, when we did not set any of the caching
@@ -1868,6 +1863,12 @@ static int change_page_attr_set_clr(unsigned
long *addr, int numpages,
         */
        cache = !!pgprot2cachemode(mask_set);
 
+       /*
+        * Check whether we really changed something:
+        */
+       if (!(cpa.flags & CPA_FLUSHTLB) && !cache)
+               goto out;
+
        /*
         * On error; flush everything to be sure.
         */

Hmm, might want to maintain the "On error; flush everything to be sure"
logic in the NP->P case as well.