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

[Miaohe Lin <linmiaohe@huawei.com>]

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 ?

> Thanks again!
>