linux-kernel - Re: [PATCH v2 01/17] mm/gup: Fixup p*_access

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Date:   Fri, 15 Dec 2017 12:38:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>, keescook@...gle.com,
        hughd@...gle.com, Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>, linux-mm@...ck.org,
        kirill.shutemov@...ux.intel.com, dan.j.williams@...el.com
Subject: Re: [PATCH v2 01/17] mm/gup: Fixup p*_access_permitted()

On Fri, Dec 15, 2017 at 11:25:29AM +0100, Peter Zijlstra wrote:
> The memory one is also clearly wrong, not having access does not a write
> fault make. If we have pte_write() set we should not do_wp_page() just
> because we don't have access. This falls under the "doing anything other
> than hard failure for !access is crazy" header.

So per the very same reasoning I think the below is warranted too; also
rename that @dirty variable, because its also wrong.

diff --git a/mm/memory.c b/mm/memory.c
index 5eb3d2524bdc..0d43b347eb0a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3987,7 +3987,7 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
 		.pgoff = linear_page_index(vma, address),
 		.gfp_mask = __get_fault_gfp_mask(vma),
 	};
-	unsigned int dirty = flags & FAULT_FLAG_WRITE;
+	unsigned int write = flags & FAULT_FLAG_WRITE;
 	struct mm_struct *mm = vma->vm_mm;
 	pgd_t *pgd;
 	p4d_t *p4d;
@@ -4013,7 +4013,7 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
 
 			/* NUMA case for anonymous PUDs would go here */
 
-			if (dirty && !pud_access_permitted(orig_pud, WRITE)) {
+			if (write && !pud_write(orig_pud)) {
 				ret = wp_huge_pud(&vmf, orig_pud);
 				if (!(ret & VM_FAULT_FALLBACK))
 					return ret;
@@ -4046,7 +4046,7 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
 			if (pmd_protnone(orig_pmd) && vma_is_accessible(vma))
 				return do_huge_pmd_numa_page(&vmf, orig_pmd);
 
-			if (dirty && !pmd_access_permitted(orig_pmd, WRITE)) {
+			if (write && !pmd_write(orig_pmd)) {
 				ret = wp_huge_pmd(&vmf, orig_pmd);
 				if (!(ret & VM_FAULT_FALLBACK))
 					return ret;


I still cannot make sense of what the intention behind these changes
were, the Changelog that went with them is utter crap, it doesn't
explain anything.