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.
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.
Thanks,
Naoya Horiguchi
