linux-kernel - Re: [PATCH v5 05/11] mm, madvise_inject_error: Let memory_failure() optionally take a page reference

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Date:   Fri, 13 Jul 2018 17:34:48 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Michal Hocko <mhocko@...e.com>, "hch@....de" <hch@....de>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jack@...e.cz" <jack@...e.cz>,
        "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH v5 05/11] mm, madvise_inject_error: Let memory_failure()
 optionally take a page reference

On Thu, Jul 12, 2018 at 11:31 PM, Naoya Horiguchi
<n-horiguchi@...jp.nec.com> 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@...e.com>
>> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>> ---
>>  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.