Re: [PATCH 3/4] mm/hugeltb: fix potential wrong gbl_reserve value for hugetlb_acct_memory()

[Mike Kravetz]

On 4/7/21 8:26 PM, Miaohe Lin wrote:
> On 2021/4/8 11:24, Miaohe Lin wrote:
>> On 2021/4/8 4:53, Mike Kravetz wrote:
>>> On 4/7/21 12:24 AM, Miaohe Lin wrote:
>>>> Hi:
>>>> On 2021/4/7 10:49, Mike Kravetz wrote:
>>>>> On 4/2/21 2:32 AM, Miaohe Lin wrote:
>>>>>> The resv_map could be NULL since this routine can be called in the evict
>>>>>> inode path for all hugetlbfs inodes. So we could have chg = 0 and this
>>>>>> would result in a negative value when chg - freed. This is unexpected for
>>>>>> hugepage_subpool_put_pages() and hugetlb_acct_memory().
>>>>>
>>>>> I am not sure if this is possible.
>>>>>
>>>>> It is true that resv_map could be NULL.  However, I believe resv map
>>>>> can only be NULL for inodes that are not regular or link inodes.  This
>>>>> is the inode creation code in hugetlbfs_get_inode().
>>>>>
>>>>>        /*
>>>>>          * Reserve maps are only needed for inodes that can have associated
>>>>>          * page allocations.
>>>>>          */
>>>>>         if (S_ISREG(mode) || S_ISLNK(mode)) {
>>>>>                 resv_map = resv_map_alloc();
>>>>>                 if (!resv_map)
>>>>>                         return NULL;
>>>>>         }
>>>>>
>>>>
>>>> Agree.
>>>>
>>>>> If resv_map is NULL, then no hugetlb pages can be allocated/associated
>>>>> with the file.  As a result, remove_inode_hugepages will never find any
>>>>> huge pages associated with the inode and the passed value 'freed' will
>>>>> always be zero.
>>>>>
>>>>
>>>> But I am confused now. AFAICS, remove_inode_hugepages() searches the address_space of
>>>> the inode to remove the hugepages while does not care if inode has associated resv_map.
>>>> How does it prevent hugetlb pages from being allocated/associated with the file if
>>>> resv_map is NULL? Could you please explain this more?
>>>>
>>>
>>> Recall that there are only two ways to get huge pages associated with
>>> a hugetlbfs file: fallocate and mmap/write fault.  Directly writing to
>>> hugetlbfs files is not supported.
>>>
>>> If you take a closer look at hugetlbfs_get_inode, it has that code to
>>> allocate the resv map mentioned above as well as the following:
>>>
>>> 		switch (mode & S_IFMT) {
>>> 		default:
>>> 			init_special_inode(inode, mode, dev);
>>> 			break;
>>> 		case S_IFREG:
>>> 			inode->i_op = &hugetlbfs_inode_operations;
>>> 			inode->i_fop = &hugetlbfs_file_operations;
>>> 			break;
>>> 		case S_IFDIR:
>>> 			inode->i_op = &hugetlbfs_dir_inode_operations;
>>> 			inode->i_fop = &simple_dir_operations;
>>>
>>> 			/* directory inodes start off with i_nlink == 2 (for "." entry) */
>>> 			inc_nlink(inode);
>>> 			break;
>>> 		case S_IFLNK:
>>> 			inode->i_op = &page_symlink_inode_operations;
>>> 			inode_nohighmem(inode);
>>> 			break;
>>> 		}
>>>
>>> Notice that only S_IFREG inodes will have i_fop == &hugetlbfs_file_operations.
>>> hugetlbfs_file_operations contain the hugetlbfs specific mmap and fallocate
>>> routines.  Hence, only files with S_IFREG inodes can potentially have
>>> associated huge pages.  S_IFLNK inodes can as well via file linking.
>>>
>>> If an inode is not S_ISREG(mode) || S_ISLNK(mode), then it will not have
>>> a resv_map.  In addition, it will not have hugetlbfs_file_operations and
>>> can not have associated huge pages.
>>>
>>
>> Many many thanks for detailed and patient explanation! :) I think I have got the idea!
>>
>>> I looked at this closely when adding commits
>>> 58b6e5e8f1ad hugetlbfs: fix memory leak for resv_map
>>> f27a5136f70a hugetlbfs: always use address space in inode for resv_map pointer
>>>
>>> I may not be remembering all of the details correctly.  Commit f27a5136f70a
>>> added the comment that resv_map could be NULL to hugetlb_unreserve_pages.
>>>
>>
>> Since we must have freed == 0 while chg == 0. Should we make this assumption explict
>> by something like below?
>>
>> WARN_ON(chg < freed);
>>
> 
> Or just a comment to avoid confusion ?
> 

Yes, add a comment to hugetlb_unreserve_pages saying that !resv_map
implies freed == 0.

It would also be helpful to check for (chg - freed) == 0 and skip the
calls to hugepage_subpool_put_pages() and hugetlb_acct_memory().  Both
of those routines may perform an unnecessary lock/unlock cycle in this
case.

A simple
	if (chg == free)
		return 0;
before the call to hugepage_subpool_put_pages would work.
-- 
Mike Kravetz