Re: [PATCH] ppc64: do page traversal if vmemmap_list not populated

[Aditya Gupta <adityag@xxxxxxxxxxxxx>]

    On 21/09/23 11:21, lijiang wrote:

On Mon, Sep 18, 2023 at 7: 34 PM lijiang <lijiang@ redhat. com> wrote: Hi, Aditya Thank you for the patch. On Mon, Sep 11, 2023 at 8: 00 PM <crash-utility-request@ redhat. com> wrote: From: Aditya Gupta <adityag@ linux. ibm. com> To: 

On Mon, Sep 18, 2023 at 7:34 PM lijiang <lijiang@xxxxxxxxxx> wrote:

Hi, Aditya

Thank you for the patch.

On Mon, Sep 11, 2023 at 8:00 PM <crash-utility-request@xxxxxxxxxx> wrote:

From: Aditya Gupta <adityag@xxxxxxxxxxxxx>
To: crash-utility@xxxxxxxxxx
Cc: Hari Bathini <hbathini@xxxxxxxxxxxxx>, Mahesh J Salgaonkar
        <mahesh@xxxxxxxxxxxxx>, Sourabh Jain <sourabhjain@xxxxxxxxxxxxx>,
        "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>
Subject: [PATCH] ppc64: do page traversal if
        vmemmap_list not populated
Message-ID: <20230911093032.419388-1-adityag@xxxxxxxxxxxxx>
Content-Type: text/plain; charset="US-ASCII"; x-default=true

Currently 'crash-tool' fails on vmcore collected on upstream kernel on
PowerPC64 with the error:

    crash: invalid kernel virtual address: 0  type: "first list entry

Presently the address translation for vmemmap addresses is done using
the vmemmap_list. But with the below commit in Linux, vmemmap_list can
be empty, in case of Radix MMU on PowerPC64

    368a0590d954: (powerpc/book3s64/vmemmap: switch radix to use a
    different vmemmap handling function)

In case vmemmap_list is empty, then it's head is NULL, which crash tries
to access and fails due to accessing NULL.

Instead of depending on 'vmemmap_list' for address translation for
vmemmap addresses, do a kernel pagetable walk to get the physical
address associated with given virtual address

Reviewed-by: Hari Bathini <hbathini@xxxxxxxxxxxxx>
Signed-off-by: Aditya Gupta <adityag@xxxxxxxxxxxxx>

---

Testing
=======

Git tree with patch applied:
https://github.com/adi-g15-ibm/crash/tree/bugzilla-203296-list-v1

This can be tested with '/proc/vmcore' as the vmcore, since makedumpfile

also fails in absence of 'vmemmap_list' in upstream linux

The fix for makedumpfile will also been sent to upstream

---
---
 ppc64.c | 47 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 33 insertions(+), 14 deletions(-)

diff --git a/ppc64.c b/ppc64.c
index fc34006f4863..1e84c5f56773 100644
--- a/ppc64.c
+++ b/ppc64.c
@@ -1280,8 +1280,30 @@ void ppc64_vmemmap_init(void)
        long backing_size, virt_addr_offset, phys_offset, list_offset;
        ulong *vmemmap_list;
        char *vmemmap_buf;
-       struct machine_specific *ms;
-       
+       struct machine_specific *ms = machdep->machspec;
+
+       ld = &list_data;
+       BZERO(ld, sizeof(struct list_data));
+
+       /*
+        * vmemmap_list is missing or set to 0 in the kernel would imply
+        * vmemmap region is mapped in the kernel pagetable. So, read vmemmap_list
+        * anyway and use the translation method accordingly.
+        */
+       readmem(symbol_value("vmemmap_list"), KVADDR, &ld->start, sizeof(void *),
+               "vmemmap_list", RETURN_ON_ERROR);
+       if (!ld->start) {
+               /*
+                * vmemmap_list is set to 0 or missing. Do kernel pagetable walk
+                * for vmemmamp address translation.
+                */
+               ms->vmemmap_list = NULL;
+               ms->vmemmap_cnt = 0;
+
+               machdep->flags |= VMEMMAP_AWARE;
+               return;
+       }
+

I would suggest putting the 'readmem(symbol_value("vmemmap_list"),...)' after the 'kernel_symbol_exists("vmemmap_list")', and also check the returned value of readmem().

Yes, I considered that, but the if condition checking for 'kernel_symbol_exists("vmemmap_list")' returns from the function if the symbol is not found, same with the readmem return value check earlier, we wanted to initialise the ms->vmemmap_list and machdep->flags before that.

The readmem used to be like this, which returns from function if readmem failed:

-       if (!readmem(symbol_value("vmemmap_list"),
-           KVADDR, &ld->start, sizeof(void *), "vmemmap_list",
-           RETURN_ON_ERROR))
-               return;

So, intentionally I put it above the if and removed the return value check, since we don't want to depend on vmemmap_list symbol being there for newer kernels anymore.
And to be future proof in case the address mapping moves to page table for Hash MMU case also, and since vmemmap_list is PowerPC specific, the symbol might also be removed.

But, if it's a coding guidelines requirement to handle the return values, I can think of how to handle the return value of readmem, any suggestions ?

And other changes are fine to me.

    Thank you for your reviews Lianbo :)

    Thanks,
    - Aditya Gupta
  

--
Crash-utility mailing list
Crash-utility@xxxxxxxxxx
https://listman.redhat.com/mailman/listinfo/crash-utility
Contribution Guidelines: https://github.com/crash-utility/crash/wiki