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

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/4/9 6:53, Mike Kravetz wrote:
> 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.
> 

Sounds good!

> 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.

This may not be really helpful because hugepage_subpool_put_pages() and hugetlb_acct_memory()
both would handle delta == 0 case without unnecessary lock/unlock cycle.
Does this make sense for you? If so, I will prepare v2 with the changes to add a comment
to hugetlb_unreserve_pages() __without__ the check for (chg - freed) == 0.

Many thanks!