Re: [PATCH v1 7/7] fs/proc/kcore: use page_offline_(freeze|unfreeze)

[David Hildenbrand <david@redhat.com>]

On 03.05.21 13:33, Mike Rapoport wrote:
> On Mon, May 03, 2021 at 12:13:45PM +0200, David Hildenbrand wrote:
>> On 03.05.21 11:28, Mike Rapoport wrote:
>>> On Mon, May 03, 2021 at 10:28:36AM +0200, David Hildenbrand wrote:
>>>> On 02.05.21 08:34, Mike Rapoport wrote:
>>>>> On Thu, Apr 29, 2021 at 02:25:19PM +0200, David Hildenbrand wrote:
>>>>>> Let's properly synchronize with drivers that set PageOffline(). Unfreeze
>>>>>> every now and then, so drivers that want to set PageOffline() can make
>>>>>> progress.
>>>>>>
>>>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>>>> ---
>>>>>>     fs/proc/kcore.c | 15 +++++++++++++++
>>>>>>     1 file changed, 15 insertions(+)
>>>>>>
>>>>>> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
>>>>>> index 92ff1e4436cb..3d7531f47389 100644
>>>>>> --- a/fs/proc/kcore.c
>>>>>> +++ b/fs/proc/kcore.c
>>>>>> @@ -311,6 +311,7 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
>>>>>>     static ssize_t
>>>>>>     read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>>>>>>     {
>>>>>> +	size_t page_offline_frozen = 0;
>>>>>>     	char *buf = file->private_data;
>>>>>>     	size_t phdrs_offset, notes_offset, data_offset;
>>>>>>     	size_t phdrs_len, notes_len;
>>>>>> @@ -509,6 +510,18 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>>>>>>     			pfn = __pa(start) >> PAGE_SHIFT;
>>>>>>     			page = pfn_to_online_page(pfn);
>>>>>
>>>>> Can't this race with page offlining for the first time we get here?
>>>>
>>>>
>>>> To clarify, we have three types of offline pages in the kernel ...
>>>>
>>>> a) Pages part of an offline memory section; the memap is stale and not
>>>> trustworthy. pfn_to_online_page() checks that. We *can* protect against
>>>> memory offlining using get_online_mems()/put_online_mems(), but usually
>>>> avoid doing so as the race window is very small (and a problem all over the
>>>> kernel we basically never hit) and locking is rather expensive. In the
>>>> future, we might switch to rcu to handle that more efficiently and avoiding
>>>> these possible races.
>>>>
>>>> b) PageOffline(): logically offline pages contained in an online memory
>>>> section with a sane memmap. virtio-mem calls these pages "fake offline";
>>>> something like a "temporary" memory hole. The new mechanism I propose will
>>>> be used to handle synchronization as races can be more severe, e.g., when
>>>> reading actual page content here.
>>>>
>>>> c) Soft offline pages: hwpoisoned pages that are not actually harmful yet,
>>>> but could become harmful in the future. So we better try to remove the page
>>>> from the page allcoator and try to migrate away existing users.
>>>>
>>>>
>>>> So page_offline_* handle "b) PageOffline()" only. There is a tiny race
>>>> between pfn_to_online_page(pfn) and looking at the memmap as we have in many
>>>> cases already throughout the kernel, to be tackled in the future.
>>>
>>> Right, but here you anyway add locking, so why exclude the first iteration?
>>
>> What we're protecting is PageOffline() below. If I didn't mess up, we should
>> always be calling page_offline_freeze() before calling PageOffline(). Or am
>> I missing something?
>   
> Somehow I was under impression we are protecting both pfn_to_online_page()
> and PageOffline().
>   
>>> BTW, did you consider something like
>>
>> Yes, I played with something like that. We'd have to handle the first
>> page_offline_freeze() freeze differently, though, and that's where things
>> got a bit ugly in my attempts.
>>
>>>
>>> 	if (page_offline_frozen++ % MAX_ORDER_NR_PAGES == 0) {
>>> 		page_offline_unfreeze();
>>> 		cond_resched();
>>> 		page_offline_freeze();
>>> 	}
>>>
>>> We don't seem to care about page_offline_frozen overflows here, do we?
>>
>> No, the buffer size is also size_t and gets incremented on a per-byte basis.
>> The variant I have right now looked the cleanest to me. Happy to hear
>> simpler alternatives.
> 
> Well, locking for the first time before the while() loop and doing
> resched-relock outside switch() would be definitely nicer, and it makes the
> last unlock unconditional.
> 
> The cost of prevention of memory offline during reads of !KCORE_RAM parts
> does not seem that significant to me, but I may be missing something.

Also true, I'll have a look if I can just simplify that.

-- 
Thanks,

David / dhildenb