linux-kernel - Re: [PATCH 3/4] mm/hugeltb: fix potential wrong gbl_reserve value for hugetlb_acct

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Date:   Wed, 7 Apr 2021 13:53:02 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     n-horiguchi@...jp.nec.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 3/4] mm/hugeltb: fix potential wrong gbl_reserve value for
 hugetlb_acct_memory()

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