Re: [PATCH 2/4] NFS: Directory page cache pages need to be locked when read

[Dai Ngo <dai.ngo@oracle.com>]

On 2/3/20 1:18 PM, Trond Myklebust wrote:
> On Mon, 2020-02-03 at 20:31 +0000, Schumaker, Anna wrote:
>> Hi Trond,
>>
>> On Sun, 2020-02-02 at 17:53 -0500, Trond Myklebust wrote:
>>> When a NFS directory page cache page is removed from the page
>>> cache,
>>> its contents are freed through a call to nfs_readdir_clear_array().
>>> To prevent the removal of the page cache entry until after we've
>>> finished reading it, we must take the page lock.
>>>
>>> Fixes: 11de3b11e08c ("NFS: Fix a memory leak in nfs_readdir")
>>> Cc: stable@vger.kernel.org # v2.6.37+
>>> Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
>>> ---
>>>   fs/nfs/dir.c | 30 +++++++++++++++++++-----------
>>>   1 file changed, 19 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
>>> index ba0d55930e8a..90467b44ec13 100644
>>> --- a/fs/nfs/dir.c
>>> +++ b/fs/nfs/dir.c
>>> @@ -705,8 +705,6 @@ int nfs_readdir_filler(void *data, struct page*
>>> page)
>>>   static
>>>   void cache_page_release(nfs_readdir_descriptor_t *desc)
>>>   {
>>> -	if (!desc->page->mapping)
>>> -		nfs_readdir_clear_array(desc->page);
>>>   	put_page(desc->page);
>>>   	desc->page = NULL;
>>>   }
>>> @@ -720,19 +718,28 @@ struct page
>>> *get_cache_page(nfs_readdir_descriptor_t
>>> *desc)
>>>   
>>>   /*
>>>    * Returns 0 if desc->dir_cookie was found on page desc-
>>>> page_index
>>> + * and locks the page to prevent removal from the page cache.
>>>    */
>>>   static
>>> -int find_cache_page(nfs_readdir_descriptor_t *desc)
>>> +int find_and_lock_cache_page(nfs_readdir_descriptor_t *desc)
>>>   {
>>>   	int res;
>>>   
>>>   	desc->page = get_cache_page(desc);
>>>   	if (IS_ERR(desc->page))
>>>   		return PTR_ERR(desc->page);
>>> -
>>> -	res = nfs_readdir_search_array(desc);
>>> +	res = lock_page_killable(desc->page);
>>>   	if (res != 0)
>>> -		cache_page_release(desc);
>>> +		goto error;
>>> +	res = -EAGAIN;
>>> +	if (desc->page->mapping != NULL) {
>>> +		res = nfs_readdir_search_array(desc);
>>> +		if (res == 0)
>>> +			return 0;
>>> +	}
>>> +	unlock_page(desc->page);
>>> +error:
>>> +	cache_page_release(desc);
>>>   	return res;
>>>   }
>>>   
>> Can you give me some guidance on how to apply this on top of Dai's v2
>> patch from
>> January 23? Right now I have the nfsi->page_index getting set before
>> the
>> unlock_page():
> Since this needs to be a stable patch, it would be preferable to apply
> them in the opposite order to avoid an extra dependency on Dai's patch.

Hi Trond, does this mean my patch won't make it to the stable backport
this time? This patch is not just a performance enhancement, but fixes
real bug, which in some cases can cause readdir to take very long time
to complete.

Thanks,
-Dai

>
> That said, since the nfsi->page_index is not a per-page variable, there
> is no need to put it under the page lock.
>
>
>> @@ -732,15 +733,24 @@ int find_cache_page(nfs_readdir_descriptor_t
>> *desc)
>>   	if (IS_ERR(desc->page))
>>   		return PTR_ERR(desc->page);
>>   
>> -	res = nfs_readdir_search_array(desc);
>> +	res = lock_page_killable(desc->page);
>>   	if (res != 0)
>>   		cache_page_release(desc);
>> +		goto error;
>> +	res = -EAGAIN;
>> +	if (desc->page->mapping != NULL) {
>> +		res = nfs_readdir_search_array(desc);
>> +		if (res == 0)
>> +			return 0;
>> +	}
>>   
>>   	dentry = file_dentry(desc->file);
>>   	inode = d_inode(dentry);
>>   	nfsi = NFS_I(inode);
>>   	nfsi->page_index = desc->page_index;
>> -
>> +	unlock_page(desc->page);
>> +error:
>> +	cache_page_release(desc);
>>   	return res;
>>   }
>>   
>>
>> Please let me know if there is a better way to handle the conflict!
>>
>> Anna
>>
>>
>>> @@ -747,7 +754,7 @@ int
>>> readdir_search_pagecache(nfs_readdir_descriptor_t
>>> *desc)
>>>   		desc->last_cookie = 0;
>>>   	}
>>>   	do {
>>> -		res = find_cache_page(desc);
>>> +		res = find_and_lock_cache_page(desc);
>>>   	} while (res == -EAGAIN);
>>>   	return res;
>>>   }
>>> @@ -786,7 +793,6 @@ int nfs_do_filldir(nfs_readdir_descriptor_t
>>> *desc)
>>>   		desc->eof = true;
>>>   
>>>   	kunmap(desc->page);
>>> -	cache_page_release(desc);
>>>   	dfprintk(DIRCACHE, "NFS: nfs_do_filldir() filling ended @
>>> cookie %Lu;
>>> returning = %d\n",
>>>   			(unsigned long long)*desc->dir_cookie, res);
>>>   	return res;
>>> @@ -832,13 +838,13 @@ int uncached_readdir(nfs_readdir_descriptor_t
>>> *desc)
>>>   
>>>   	status = nfs_do_filldir(desc);
>>>   
>>> + out_release:
>>> +	nfs_readdir_clear_array(desc->page);
>>> +	cache_page_release(desc);
>>>    out:
>>>   	dfprintk(DIRCACHE, "NFS: %s: returns %d\n",
>>>   			__func__, status);
>>>   	return status;
>>> - out_release:
>>> -	cache_page_release(desc);
>>> -	goto out;
>>>   }
>>>   
>>>   /* The file offset position represents the dirent entry number.  A
>>> @@ -903,6 +909,8 @@ static int nfs_readdir(struct file *file,
>>> struct
>>> dir_context *ctx)
>>>   			break;
>>>   
>>>   		res = nfs_do_filldir(desc);
>>> +		unlock_page(desc->page);
>>> +		cache_page_release(desc);
>>>   		if (res < 0)
>>>   			break;
>>>   	} while (!desc->eof);