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From: "yukuai (C)" <yukuai3@huawei.com>
To: Hugh Dickins <hughd@google.com>, Greg KH <gregkh@linuxfoundation.org>
Cc: <stable@vger.kernel.org>, <viro@zeniv.linux.org.uk>,
	<yi.zhang@huawei.com>, <zhengbin13@huawei.com>,
	<houtao1@huawei.com>
Subject: Re: [4.19.y PATCH] tmpfs: fix unable to remount nr_inodes from limited to unlimited
Date: Thu, 5 Dec 2019 09:41:39 +0800
Message-ID: <264fe57a-5b6b-cade-f31f-bb4861668d18@huawei.com> (raw)
In-Reply-To: <alpine.LSU.2.11.1912041142060.1591@eggly.anvils>



On 2019/12/5 4:13, Hugh Dickins wrote:
> On Wed, 4 Dec 2019, Greg KH wrote:
>> On Wed, Dec 04, 2019 at 09:11:37PM +0800, yu kuai wrote:
>>> tmpfs support 'size', 'nr_blocks' and 'nr_inodes' mount options. mount or
>>> remount them to zero means unlimited. 'size' and 'br_blocks' can remount
>>> from limited to unlimited, while 'nr_inodes' can't.
>>>
>>> The problem is fixed since upstream commit 0b5071dd323d ("
>>> shmem_parse_options(): use a separate structure to keep the results"). But
>>> in order to backport it, the amount of related patches need to backport is
>>> huge.
>>>
>>> So, I made some local changes to fix the problem.
>>>
>>> Signed-off-by: yu kuai <yukuai3@huawei.com>
>>> ---
>>>   mm/shmem.c | 7 +++++--
>>>   1 file changed, 5 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>> index 3c8742655756..966fc69ee8fb 100644
>>> --- a/mm/shmem.c
>>> +++ b/mm/shmem.c
>>> @@ -3444,7 +3444,7 @@ static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
>>>   	inodes = sbinfo->max_inodes - sbinfo->free_inodes;
>>>   	if (percpu_counter_compare(&sbinfo->used_blocks, config.max_blocks) > 0)
>>>   		goto out;
>>> -	if (config.max_inodes < inodes)
>>> +	if (config.max_inodes && config.max_inodes < inodes)
>>>   		goto out;
>>>   	/*
>>>   	 * Those tests disallow limited->unlimited while any are in use;
>>> @@ -3460,7 +3460,10 @@ static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
>>>   	sbinfo->huge = config.huge;
>>>   	sbinfo->max_blocks  = config.max_blocks;
>>>   	sbinfo->max_inodes  = config.max_inodes;
>>> -	sbinfo->free_inodes = config.max_inodes - inodes;
>>> +	if (!config.max_inodes)
>>> +		sbinfo->free_inodes = 0;
>>> +	else
>>> +		sbinfo->free_inodes = config.max_inodes - inodes;
>>>   
>>>   	/*
>>>   	 * Preserve previous mempolicy unless mpol remount option was specified.
>>> -- 
>>> 2.17.2
>>>
>>
>> Hm, sorry about my bot, this looked like an odd one-off patch.
>>
>> What about 5.3.y, should this patch also go there as well?
>>
>> But is it really an issue as this is a new "feature" that 5.4 now has,
>> can't you just use 5.4.y if you need this type of thing?  It's never
>> worked in the past, right?
> 
> Yes, please ignore this for stable, Greg: it appears to be a new feature
> in 5.4: one that I should have noticed when testing, but failed to do so
> (and it may even be something that I foisted unthinkingly on Al when
> suggesting mods to his and David's originals).
> 
> Yu Kuai: many thanks for noticing and reporting this, I was unconscious
> of changing behavior here.  Notice how the 5.4 shmem_reconfigure() still
> has a comment above it saying "we disallow change from limited->unlimited
> blocks/inodes while any are in use" - and root inode is always in use.
> Notice how your 4.19 patch does nothing for max_blocks, so remounting
> with nr_blocks=0 will still fail, once a non-empty file has been created.
Thank you for your response, I missed that in my test.
> 
> I agree that it's not obvious why limited->unlimited needs to fail,
> and perhaps a nice (worthwhile?) little enhancement to allow that;
> but it was unintentional, and now (but not today) I have to go back
> to remind myself why 2.6.13 implemented it with that restriction,
> and whether there are any fixes needed to the new behavior in 5.4
> (at the least, we ought to fix that comment in 5.5).

I was confused about the comment in 5.5: "we disallow change from 
limited->unlimited blocks/inodes while any are in use", it seems change 
from limited to unlimited will always succed. Maybe this is
more appropriate: "Note that we allow change from limited to unlimited 
blocks/inodes. But we disallow change from unlimited->limited, because 
in that case we have no record of how much is already in use. And we 
disallow change blocks/inodes to less than the amount that is aready in use"

Thanks
Yu Kuai


  reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-04 13:11 yu kuai
2019-12-04 17:31 ` Greg KH
2019-12-04 17:33 ` Greg KH
2019-12-04 20:13   ` Hugh Dickins
2019-12-05  1:41     ` yukuai (C) [this message]
2019-12-05  1:22   ` yukuai (C)

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