On Thu, Jul 12, 2018 at 11:31 PM, Naoya Horiguchi
<n-horiguchi@xxxxxxxxxxxxx> wrote:
> Hello Dan,
>
> On Wed, Jul 04, 2018 at 02:40:49PM -0700, Dan Williams wrote:
>> The madvise_inject_error() routine uses get_user_pages() to lookup the
>> pfn and other information for injected error, but it does not release
>> that pin. The assumption is that failed pages should be taken out of
>> circulation.
>>
>> However, for dax mappings it is not possible to take pages out of
>> circulation since they are 1:1 physically mapped as filesystem blocks,
>> or device-dax capacity. They also typically represent persistent memory
>> which has an error clearing capability.
>>
>> In preparation for adding a special handler for dax mappings, shift the
>> responsibility of taking the page reference to memory_failure(). I.e.
>> drop the page reference and do not specify MF_COUNT_INCREASED to
>> memory_failure().
>>
>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>> mm/madvise.c | 18 +++++++++++++++---
>> 1 file changed, 15 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/madvise.c b/mm/madvise.c
>> index 4d3c922ea1a1..b731933dddae 100644
>> --- a/mm/madvise.c
>> +++ b/mm/madvise.c
>> @@ -631,11 +631,13 @@ static int madvise_inject_error(int behavior,
>>
>>
>> for (; start < end; start += PAGE_SIZE << order) {
>> + unsigned long pfn;
>> int ret;
>>
>> ret = get_user_pages_fast(start, 1, 0, &page);
>> if (ret != 1)
>> return ret;
>> + pfn = page_to_pfn(page);
>>
>> /*
>> * When soft offlining hugepages, after migrating the page
>> @@ -651,17 +653,27 @@ static int madvise_inject_error(int behavior,
>>
>> if (behavior == MADV_SOFT_OFFLINE) {
>> pr_info("Soft offlining pfn %#lx at process virtual address %#lx\n",
>> - page_to_pfn(page), start);
>> + pfn, start);
>>
>> ret = soft_offline_page(page, MF_COUNT_INCREASED);
>> if (ret)
>> return ret;
>> continue;
>> }
>> +
>> pr_info("Injecting memory failure for pfn %#lx at process virtual address %#lx\n",
>> - page_to_pfn(page), start);
>> + pfn, start);
>> +
>> + ret = memory_failure(pfn, 0);
>> +
>> + /*
>> + * Drop the page reference taken by get_user_pages_fast(). In
>> + * the absence of MF_COUNT_INCREASED the memory_failure()
>> + * routine is responsible for pinning the page to prevent it
>> + * from being released back to the page allocator.
>> + */
>> + put_page(page);
>>
>> - ret = memory_failure(page_to_pfn(page), MF_COUNT_INCREASED);
>
> MF_COUNT_INCREASED means that the page refcount for memory error handling
> is taken by the caller so you don't have to take one inside memory_failure().
> So this code don't keep with the definition, then another refcount can be
> taken in memory_failure() in normal LRU page's case for example.
> As a result the error message "Memory failure: %#lx: %s still referenced by
> %d users\n" will be dumped in page_action().
>
> So if you want to put put_page() in madvise_inject_error(), I think that
>
> put_page(page);
> ret = memory_failure(pfn, 0);
>
> can be acceptable because the purpose of get_user_pages_fast() here is
> just getting pfn, and the refcount itself is not so important.
> IOW, memory_failure() is called only with pfn which never changes depending
> on the page's status.
Ok, I'll resend with the put_page() moved before memory_failure() to
make it more clear that memory_failure() is responsible for taking its
own reference and that there is no dependency to hold the reference in
madvise_inject_error().
> In production system memory_failure() is called via machine check code
> without taking any pagecount, so I don't think the this injection interface
> is properly mocking the real thing. So I'm feeling that this flag will be
> wiped out at some point.
Ok, makes sense.
