linux-kernel - Re: [PATCH 5/5] hugetlbfs: fix confusing hugetlbfs stat

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Message-ID: <e6e2de0d-ee72-4572-75c5-02cb27e7ae95@huawei.com>
Date:   Sat, 23 Jul 2022 10:56:00 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     <akpm@...ux-foundation.org>, <songmuchun@...edance.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] hugetlbfs: fix confusing hugetlbfs stat

On 2022/7/23 6:55, Mike Kravetz wrote:
> On 07/22/22 14:38, Miaohe Lin wrote:
>> On 2022/7/22 8:28, Mike Kravetz wrote:
>>> On 07/21/22 21:16, Miaohe Lin wrote:
>>>> When size option is not specified, f_blocks, f_bavail and f_bfree will be
>>>> set to -1 instead of 0. Likewise, when nr_inodes is not specified, f_files
>>>> and f_ffree will be set to -1 too. Check max_hpages and max_inodes against
>>>> -1 first to make sure 0 is reported for max/free/used when no limit is set
>>>> as the comment states.
>>>
>>> Just curious, where are you seeing values reported as -1?  The check
>>
>> From the standard statvfs() function.
>>
>>> for sbinfo->spool was supposed to handle these cases.  Seems like it
>>
>> sbinfo->spool could be created when ctx->max_hpages == -1 while
>> ctx->min_hpages != -1 in hugetlbfs_fill_super.
>>
>>> should handle the max_hpages == -1 case.  But, it doesn't look like it
>>> considers the max_inodes == -1 case.
>>>
>>> If I create/mount a hugetlb filesystem without specifying size or nr_inodes,
>>> df seems to report zero instead of -1.
>>>
>>> Just want to understand the reasoning behind the change.
> 
> Thanks for the additional information (and test program)!
> 
>>>From the hugetlbfs documentation:
> "If the ``size``, ``min_size`` or ``nr_inodes`` option is not provided on
>  command line then no limits are set."
> 
> So, having those values set to -1 indicates there is no limit set.
> 
> With this change, 0 is reported for the case where there is no limit set as
> well as the case where the max value is 0.

IMHO, 0 should not be a valid max value otherwise there will be no hugetlb pages
to use. It should mean there's no limit. But maybe I'm wrong.

> 
> There may be some value in reporting -1 as is done today.

There still be a inconsistency:

If the ``size`` and ``min_size`` isn't specified, then reported max value is 0.
But if ``min_size`` is specified while ``size`` isn't specified, the reported
max value is -1.

> 
> To be honest, I am not sure what is the correct behavior here.  Unless
> there is a user visible issue/problem, I am hesitant to change.  Other
> opinions are welcome.

Yes, it might be better to keep it as is. Maybe we could change the comment to
reflect what the current behavior is like below?

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 44da9828e171..f03b1a019cc0 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -1080,7 +1080,7 @@ static int hugetlbfs_statfs(struct dentry *dentry, struct kstatfs *buf)
        buf->f_bsize = huge_page_size(h);
        if (sbinfo) {
                spin_lock(&sbinfo->stat_lock);
-               /* If no limits set, just report 0 for max/free/used
+               /* If no limits set, just report 0 or -1 for max/free/used
                 * blocks, like simple_statfs() */
                if (sbinfo->spool) {
                        spin_lock_irq(&sbinfo->spool->lock);

> 

No strong opinion to keep this patch or above change. Many thanks for your comment and reply. :)