Re: [PATCH 1/4] fs: fix data invalidation in the cleancache during direct IO

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 04/18/2017 10:38 PM, Ross Zwisler wrote:
> On Fri, Apr 14, 2017 at 05:07:50PM +0300, Andrey Ryabinin wrote:
>> Some direct write fs hooks call invalidate_inode_pages2[_range]()
>> conditionally iff mapping->nrpages is not zero. If page cache is empty,
>> buffered read following after direct IO write would get stale data from
>> the cleancache.
>>
>> Also it doesn't feel right to check only for ->nrpages because
>> invalidate_inode_pages2[_range] invalidates exceptional entries as well.
>>
>> Fix this by calling invalidate_inode_pages2[_range]() regardless of nrpages
>> state.
>>
>> Fixes: c515e1fd361c ("mm/fs: add hooks to support cleancache")
>> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
>> ---
> <>
>> diff --git a/fs/dax.c b/fs/dax.c
>> index 2e382fe..1e8cca0 100644
>> --- a/fs/dax.c
>> +++ b/fs/dax.c
>> @@ -1047,7 +1047,7 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>>  	 * into page tables. We have to tear down these mappings so that data
>>  	 * written by write(2) is visible in mmap.
>>  	 */
>> -	if ((iomap->flags & IOMAP_F_NEW) && inode->i_mapping->nrpages) {
>> +	if ((iomap->flags & IOMAP_F_NEW)) {
>>  		invalidate_inode_pages2_range(inode->i_mapping,
>>  					      pos >> PAGE_SHIFT,
>>  					      (end - 1) >> PAGE_SHIFT);
> 
> tl;dr: I think the old code is correct, and that you don't need this change.
> 
> This should be harmless, but could slow us down a little if we keep
> calling invalidate_inode_pages2_range() without really needing to.  Really for
> DAX I think we need to call invalidate_inode_page2_range() only if we have
> zero pages mapped over the place where we are doing I/O, which is why we check
> nrpages.
> 

Check for ->nrpages only looks strange, because invalidate_inode_pages2_range() also
invalidates exceptional radix tree entries. Is that correct that we invalidate
exceptional entries only if ->nrpages > 0 and skip invalidation otherwise?


> Is DAX even allowed to be used at the same time as cleancache?  From a brief
> look at Documentation/vm/cleancache.txt, it seems like these two features are
> incompatible.  With DAX we already are avoiding the page cache completely.

tl;dr: I think you're right.

cleancache may store any PageUptodate && PageMappedToDisk page evicted from page cache (see __delete_from_page_cache)
DAX deletes hole page via __delete_from_page_cache(), but I don't see we mark such page as Uptodate or MappedToDisk
so it will never go into the cleancache.

Latter cleancache_get_page() (e.g. it's called from mpage_readpages() which is called from blkdev_read_pages())
I assume that DAX doesn't use a_ops->readpages() method so cleancache_get_page() is never called from DAX.


> Anyway, I don't see how this change in DAX can save us from a data corruption
> (which is what you're seeing, right?), and I think it could slow us down, so
> I'd prefer to leave things as they are.
> 

I'll remove this hunk from v2.

Thanks.