Re: [PATCH] f2fs: allow readpages with NULL file pointer

[Chao Yu <chao@kernel.org>]

Hi Xiang Gao,

On 2017/9/22 9:54, gaoxiang (P) wrote:
> Hi Chao,
> 
> Thanks for your reply.
> This patch originates from the our sdcardfs compatible case-insensitive lookup acceleration.
> 
> For the customized case-insensitive create operation since f2fs has no hash and it needs to decrypt all the dirent name,
> it is recommended to read ahead the whole dir data in advance.
> 
> However I found f2fs cannot use page_cache_sync_readahead with null @file pointer as the other common Linux file system,
> this feature is something like a  bonus and I know that Linux has no written words that we should support null @file pointer, yet
> page_cache_sync_readahead is an EXPORT_SYMBOL_GPL and could be used by other non-standard 3rd kernel modules and
> of course I think it is really no problem.

Looks like some filesystems store private data in file->private_data for some
purpose during ->open, and will use the data in late ->readpages. So passing
@file with NULL value will simply cause kernel bug for these filesystems.

As I checked, most generic local filesystems just use mapping->host instead of
file->f_mapping->host, so look this patch again, following generic filesystem
looks not bad. ;)

How about just changing file->f_mapping->host to mapping->host in out patch?

Thanks,

> 
> Anyway, if it has no use to the community, I will apply it only on the internal branch.
> And I will also find some common examples of data page readahead. :)
> 
> Thanks,
> 
>> -----Original Message-----
>> From: Chao Yu [mailto:chao@kernel.org]
>> Sent: Thursday, September 21, 2017 10:33 PM
>> To: gaoxiang (P) <gaoxiang25@huawei.com>; jaegeuk@kernel.org; Yuchao (T)
>> <yuchao0@huawei.com>
>> Cc: linux-f2fs-devel@lists.sourceforge.net
>> Subject: Re: [f2fs-dev] [PATCH] f2fs: allow readpages with NULL file pointer
>>
>> On 2017/9/21 13:00, gaoxiang (P) wrote:
>>> Keep in line with the other Linux file system implementations since
>>> page_cache_sync_readahead supports NULL file pointer, and thus we can
>>> readahead data by f2fs itself without file opening (something like the
>>> btrfs behavior).
>>
>> Let's keep what it is until there is an example doing readahead passing @file
>> with NULL.
>>
>> Thanks,
>>
>>>
>>> Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
>>> ---
>>>  fs/f2fs/data.c | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c index 95f30f0..afa12f1
>>> 100644
>>> --- a/fs/f2fs/data.c
>>> +++ b/fs/f2fs/data.c
>>> @@ -1333,9 +1333,11 @@ static int f2fs_read_data_pages(struct file *file,
>>>  			struct address_space *mapping,
>>>  			struct list_head *pages, unsigned nr_pages)  {
>>> -	struct inode *inode = file->f_mapping->host;
>>> +	struct inode *inode = mapping->host;
>>>  	struct page *page = list_last_entry(pages, struct page, lru);
>>>
>>> +	if (likely(file != NULL))
>>> +		BUG_ON(file->f_mapping != mapping);
>>>  	trace_f2fs_readpages(inode, page, nr_pages);
>>>
>>>  	/* If the file has inline data, skip readpages */
>>>

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