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

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.
--
Mike Kravetz

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