Hi Anshuman,
On 7/12/21 2:14 PM, Anshuman Khandual wrote:
Though I have not jumped into the details for all individual
patches here but still have some high level questions below.
On 7/6/21 11:47 AM, Gavin Shan wrote:
There are couple of issues with current implementations and this series
tries to resolve the issues:
(a) All needed information are scattered in variables, passed to various
test functions. The code is organized in pretty much relaxed fashion.
All these variables are first prepared in debug_vm_pgtable(), before
getting passed into respective individual test functions. Also these
test functions receive only the required number of variables not all.
Adding a structure that captures all test parameters at once before
passing them down will be unnecessary. I am still wondering what will
be the real benefit of this large code churn ?
Thanks for your review. There are couple of reasons to have "struct vm_pgtable_debug".
(1) With the struct, the old and new implementation can coexist. In this way,
the patches in this series can be stacked up easily.
(2) I think passing single struct to individual test functions improves the
code readability. Besides, it also makes the empty stubs simplified.
(3) The code can be extended easily if we need in future.
(b) The page isn't allocated from buddy during page table entry modifying
tests. The page can be invalid, conflicting to the implementations
of set_{pud, pmd, pte}_at() on ARM64. The target page is accessed
so that the iCache can be flushed when execution permission is given
on ARM64. Besides, the target page can be unmapped and access to
it causes kernel crash.
Using 'start_kernel' based method for struct page usage, enabled this
test to run on platforms which might not have enough memory required
for various individual test functions. This method is not a problem for
tests that just need an aligned pfn (which creates a page table entry)
not a real struct page.
But not allocating and owning the struct page might be problematic for
tests that expect a real struct page and transform its state via set_
{pud, pmd, pte}_at() functions as reported here.
Yeah, I totally agree. The series follows what you explained: Except the
test cases where set_{pud, pmd, pte}_at() is used, the allocated page
is used. For other test cases, 'start_kernel' based PFN is used as before.
"struct vm_pgtable_debug" is introduced to address issue (a). For issue
(b), the used page is allocated from buddy in page table entry modifying
tests. The corresponding tets will be skipped if we fail to allocate the
(huge) page. For other test cases, the original page around to kernel
symbol (@start_kernel) is still used.
For all basic pfn requiring tests, existing 'start_kernel' based method
should continue but allocate a struct page for other tests which change
the passed struct page. Skipping the tests when allocation fails is the
right thing to do.
Yes, it's exactly what this series does. Hope you can jump into the details
when you get a chance :)
Thanks,
Gavin
