On Mon, Sep 16, 2019 at 09:35:21AM +0000, Justin He (Arm Technology China) wrote:

Hi Kirill

quoted

-----Original Message-----
From: Kirill A. Shutemov <redacted>
Sent: 2019年9月16日 17:16
To: Justin He (Arm Technology China) <redacted>
Cc: Catalin Marinas <Catalin.Marinas@arm.com>; Will Deacon
[off-list ref]; Mark Rutland [off-list ref]; James Morse
[off-list ref]; Marc Zyngier [off-list ref]; Matthew
Wilcox [off-list ref]; Kirill A. Shutemov
[off-list ref]; linux-arm-kernel@lists.infradead.org;
linux-kernel@vger.kernel.org; linux-mm@kvack.org; Punit Agrawal
[off-list ref]; Anshuman Khandual
[off-list ref]; Jun Yao [off-list ref];
Alex Van Brunt [off-list ref]; Robin Murphy
[off-list ref]; Thomas Gleixner [off-list ref];
Andrew Morton [off-list ref]; Jérôme Glisse
[off-list ref]; Ralph Campbell [off-list ref];
hejianet@gmail.com
Subject: Re: [PATCH v3 2/2] mm: fix double page fault on arm64 if PTE_AF
is cleared

On Sat, Sep 14, 2019 at 12:32:39AM +0800, Jia He wrote:

quoted

When we tested pmdk unit test [1] vmmalloc_fork TEST1 in arm64 guest,

there

quoted

will be a double page fault in __copy_from_user_inatomic of

cow_user_page.

quoted

Below call trace is from arm64 do_page_fault for debugging purpose
[  110.016195] Call trace:
[  110.016826]  do_page_fault+0x5a4/0x690
[  110.017812]  do_mem_abort+0x50/0xb0
[  110.018726]  el1_da+0x20/0xc4
[  110.019492]  __arch_copy_from_user+0x180/0x280
[  110.020646]  do_wp_page+0xb0/0x860
[  110.021517]  __handle_mm_fault+0x994/0x1338
[  110.022606]  handle_mm_fault+0xe8/0x180
[  110.023584]  do_page_fault+0x240/0x690
[  110.024535]  do_mem_abort+0x50/0xb0
[  110.025423]  el0_da+0x20/0x24

The pte info before __copy_from_user_inatomic is (PTE_AF is cleared):
[ffff9b007000] pgd=000000023d4f8003, pud=000000023da9b003,

pmd=000000023d4b3003, pte=360000298607bd3

quoted

As told by Catalin: "On arm64 without hardware Access Flag, copying

from

quoted

user will fail because the pte is old and cannot be marked young. So we
always end up with zeroed page after fork() + CoW for pfn mappings. we
don't always have a hardware-managed access flag on arm64."

This patch fix it by calling pte_mkyoung. Also, the parameter is
changed because vmf should be passed to cow_user_page()

[1]

https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork

quoted

Reported-by: Yibo Cai <redacted>
Signed-off-by: Jia He <redacted>
---
 mm/memory.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index e2bb51b6242e..a64af6495f71 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -118,6 +118,13 @@ int randomize_va_space __read_mostly =
                                    2;
 #endif

+#ifndef arch_faults_on_old_pte
+static inline bool arch_faults_on_old_pte(void)
+{
+   return false;
+}
+#endif
+
 static int __init disable_randmaps(char *s)
 {
    randomize_va_space = 0;

@@ -2140,7 +2147,8 @@ static inline int pte_unmap_same(struct

mm_struct *mm, pmd_t *pmd,

quoted

    return same;
 }

-static inline void cow_user_page(struct page *dst, struct page *src,

unsigned long va, struct vm_area_struct *vma)

quoted

+static inline void cow_user_page(struct page *dst, struct page *src,
+                           struct vm_fault *vmf)
 {
    debug_dma_assert_idle(src);

@@ -2152,20 +2160,32 @@ static inline void cow_user_page(struct page

*dst, struct page *src, unsigned lo

quoted

     */
    if (unlikely(!src)) {
            void *kaddr = kmap_atomic(dst);
-           void __user *uaddr = (void __user *)(va & PAGE_MASK);
+           void __user *uaddr = (void __user *)(vmf->address &

PAGE_MASK);

quoted

+           pte_t entry;

            /*
             * This really shouldn't fail, because the page is there
             * in the page tables. But it might just be unreadable,
             * in which case we just give up and fill the result with
-            * zeroes.
+            * zeroes. If PTE_AF is cleared on arm64, it might
+            * cause double page fault. So makes pte young here
             */
+           if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte))

{

quoted

+                   spin_lock(vmf->ptl);
+                   entry = pte_mkyoung(vmf->orig_pte);

Should't you re-validate that orig_pte after re-taking ptl? It can be
stale by now.

Thanks, do you mean flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte))
before pte_mkyoung?

No. You need to check pte_same(*vmf->pte, vmf->orig_pte) before modifying
anything and bail out if *vmf->pte has changed under you.

-- 
 Kirill A. Shutemov

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