Re: [RFC] remove unnecessary condition in remove_inode_hugepages

From: Mike Kravetz
Date: Sat Sep 24 2016 - 20:07:41 EST

On 09/23/2016 07:56 PM, zhong jiang wrote:
> On 2016/9/24 1:19, Mike Kravetz wrote:
>> On 09/22/2016 06:53 PM, zhong jiang wrote:
>>> At present, we need to call hugetlb_fix_reserve_count when hugetlb_unrserve_pages fails,
>>> and PagePrivate will decide hugetlb reserves counts.
>>> we obtain the page from page cache. and use page both lock_page and mutex_lock.
>>> alloc_huge_page add page to page chace always hold lock page, then bail out clearpageprivate
>>> before unlock page.
>>> but I' m not sure it is right or I miss the points.
>> Let me try to explain the code you suggest is unnecessary.
>> The PagePrivate flag is used in huge page allocation/deallocation to
>> indicate that the page was globally reserved. For example, in
>> dequeue_huge_page_vma() there is this code:
>> if (page) {
>> if (avoid_reserve)
>> break;
>> if (!vma_has_reserves(vma, chg))
>> break;
>> SetPagePrivate(page);
>> h->resv_huge_pages--;
>> break;
>> }
>> and in free_huge_page():
>> restore_reserve = PagePrivate(page);
>> ClearPagePrivate(page);
>> .
>> <snip>
>> .
>> if (restore_reserve)
>> h->resv_huge_pages++;
>> This helps maintains the global huge page reserve count.
>> In addition to the global reserve count, there are per VMA reservation
>> structures. Unfortunately, these structures have different meanings
>> depending on the context in which they are used.
>> If there is a VMA reservation entry for a page, and the page has not
>> been instantiated in the VMA this indicates there is a huge page reserved
>> and the global resv_huge_pages count reflects that reservation. Even
>> if a page was not reserved, a VMA reservation entry is added when a page
>> is instantiated in the VMA.
>> With that background, let's look at the existing code/proposed changes.
> Clearly.
>>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>>> index 4ea71eb..010723b 100644
>>> --- a/fs/hugetlbfs/inode.c
>>> +++ b/fs/hugetlbfs/inode.c
>>> @@ -462,14 +462,12 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>>> * the page, note PagePrivate which is used in case
>>> * of error.
>>> */
>>> - rsv_on_error = !PagePrivate(page);
>> This rsv_on_error flag indicates that when the huge page was allocated,
> yes
>> it was NOT counted against the global reserve count. So, when
>> remove_huge_page eventually calls free_huge_page(), the global count
>> resv_huge_pages is not incremented. So far, no problem.
> but the page comes from the page cache. if it is. it should implement
> ClearPageprivate(page) when lock page. This condition always true.
> The key point is why it need still check the PagePrivate(page) when page from
> page cache and hold lock.

You are correct. My apologies for not seeing your point in the original

When the huge page is added to the page cache (huge_add_to_page_cache),
the Page Private flag will be cleared. Since this code
(remove_inode_hugepages) will only be called for pages in the page cache,
PagePrivate(page) will always be false.

The comments in this area should be changed along with the code.

Mike Kravetz

> Thanks you
> zhongjiang
>>> remove_huge_page(page);
>>> freed++;
>>> if (!truncate_op) {
>>> if (unlikely(hugetlb_unreserve_pages(inode,
>>> next, next + 1, 1)))
>> We now have this VERY unlikely situation that hugetlb_unreserve_pages fails.
>> This means that the VMA reservation entry for the page was not removed.
>> So, we are in a bit of a mess. The page has already been removed, but the
>> VMA reservation entry can not. This LOOKS like there is a reservation for
>> the page in the VMA reservation structure. But, the global count
>> resv_huge_pages does not reflect this reservation.
>> If we do nothing, when the VMA is eventually removed the VMA reservation
>> structure will be completely removed and the global count resv_huge_pages
>> will be decremented for each entry in the structure. Since, there is a
>> VMA reservation entry without a corresponding global count, the global
>> count will be one less than it should (will eventually go to -1).
>> To 'fix' this, hugetlb_fix_reserve_counts is called. In this case, it will
>> increment the global count so that it is consistent with the entries in
>> the VMA reservation structure.
>> This is all quite confusing and really unlikely to happen. I tried to
>> explain in code comments:
>> Before removing the page:
>> /*
>> * We must free the huge page and remove from page
>> * cache (remove_huge_page) BEFORE removing the
>> * region/reserve map (hugetlb_unreserve_pages). In
>> * rare out of memory conditions, removal of the
>> * region/reserve map could fail. Before free'ing
>> * the page, note PagePrivate which is used in case
>> * of error.
>> */
>> And, the routine hugetlb_fix_reserve_counts:
>> /*
>> * A rare out of memory error was encountered which prevented removal of
>> * the reserve map region for a page. The huge page itself was free'ed
>> * and removed from the page cache. This routine will adjust the subpool
>> * usage count, and the global reserve count if needed. By incrementing
>> * these counts, the reserve map entry which could not be deleted will
>> * appear as a "reserved" entry instead of simply dangling with incorrect
>> * counts.
>> */