Re: [PATCH RFC] iomap: introduce IOMAP_TAIL

[Chao Yu <yuchao0@huawei.com>]

On 2019/7/1 17:49, Andreas Grünbacher wrote:
> Am Mo., 1. Juli 2019 um 10:04 Uhr schrieb Gao Xiang <gaoxiang25@huawei.com>:
>> On 2019/7/1 14:40, Chao Yu wrote:
>>> Hi Xiang,
>>>
>>> On 2019/6/29 17:34, Gao Xiang wrote:
>>>> Hi Chao,
>>>>
>>>> On 2019/6/29 15:30, Chao Yu wrote:
>>>>> Some filesystems like erofs/reiserfs have the ability to pack tail
>>>>> data into metadata, however iomap framework can only support mapping
>>>>> inline data with IOMAP_INLINE type, it restricts that:
>>>>> - inline data should be locating at page #0.
>>>>> - inline size should equal to .i_size
>>>>> So we can not use IOMAP_INLINE to handle tail-packing case.
>>>>>
>>>>> This patch introduces new mapping type IOMAP_TAIL to map tail-packed
>>>>> data for further use of erofs.
>>>>>
>>>>> Signed-off-by: Chao Yu <yuchao0@huawei.com>
>>>>> ---
>>>>>  fs/iomap.c            | 22 ++++++++++++++++++++++
>>>>>  include/linux/iomap.h |  1 +
>>>>>  2 files changed, 23 insertions(+)
>>>>>
>>>>> diff --git a/fs/iomap.c b/fs/iomap.c
>>>>> index 12654c2e78f8..ae7777ce77d0 100644
>>>>> --- a/fs/iomap.c
>>>>> +++ b/fs/iomap.c
>>>>> @@ -280,6 +280,23 @@ iomap_read_inline_data(struct inode *inode, struct page *page,
>>>>>     SetPageUptodate(page);
>>>>>  }
>>>>>
>>>>> +static void
>>>>> +iomap_read_tail_data(struct inode *inode, struct page *page,
>>>>> +           struct iomap *iomap)
>>>>> +{
>>>>> +   size_t size = i_size_read(inode) & (PAGE_SIZE - 1);
>>>>> +   void *addr;
>>>>> +
>>>>> +   if (PageUptodate(page))
>>>>> +           return;
>>>>> +
>>>>> +   addr = kmap_atomic(page);
>>>>> +   memcpy(addr, iomap->inline_data, size);
>>>>> +   memset(addr + size, 0, PAGE_SIZE - size);
>>>>
>>>> need flush_dcache_page(page) here for new page cache page since
>>>> it's generic iomap code (althrough not necessary for x86, arm), I am not sure...
>>>> see commit d2b2c6dd227b and c01778001a4f...
>>>
>>> Thanks for your reminding, these all codes were copied from
>>> iomap_read_inline_data(), so I think we need a separated patch to fix this issue
>>> if necessary.
>>
>> Yes, just a reminder, it is good as it-is.
> 
> Not sure if that means that IOMAP_INLINE as is works for you now. In
> any case, if the inline data isn't transparently copied into the page
> cache at index 0, memory-mapped I/O isn't going to work.
> 
> The code further assumes that "packed" files consist of exactly one
> IOMAP_INLINE mapping; no IOMAP_MAPPED or other mappings may follow. Is
> it that assumption that's causing you trouble? If so, what's the

Yes, that's the problem we met.

> layout at the filesystem level you want to support?

The layout which can support tail-merge one, it means if the data locating at
the tail of file has small enough size, we can inline the tail data into
metadata area. e.g.:

IOMAP_MAPPED [0, 8192]
IOMAP_INLINE (or maybe IOMAP_TAIL) [8192, 8200]

Thanks,

> 
> Thanks,
> Andreas
> 
>> Thanks,
>> Gao Xiang
>>
>>>
>>> Thanks,
>>>
>>>>
>>>> Thanks,
>>>> Gao Xiang
>>>>
>>>>> +   kunmap_atomic(addr);
>>>>> +   SetPageUptodate(page);
>>>>> +}
>>>>> +
>>>>>  static loff_t
>>>>>  iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>>>>>             struct iomap *iomap)
>>>>> @@ -298,6 +315,11 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>>>>>             return PAGE_SIZE;
>>>>>     }
>>>>>
>>>>> +   if (iomap->type == IOMAP_TAIL) {
>>>>> +           iomap_read_tail_data(inode, page, iomap);
>>>>> +           return PAGE_SIZE;
>>>>> +   }
>>>>> +
>>>>>     /* zero post-eof blocks as the page may be mapped */
>>>>>     iomap_adjust_read_range(inode, iop, &pos, length, &poff, &plen);
>>>>>     if (plen == 0)
>>>>> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
>>>>> index 2103b94cb1bf..7e1ee48e3db7 100644
>>>>> --- a/include/linux/iomap.h
>>>>> +++ b/include/linux/iomap.h
>>>>> @@ -25,6 +25,7 @@ struct vm_fault;
>>>>>  #define IOMAP_MAPPED       0x03    /* blocks allocated at @addr */
>>>>>  #define IOMAP_UNWRITTEN    0x04    /* blocks allocated at @addr in unwritten state */
>>>>>  #define IOMAP_INLINE       0x05    /* data inline in the inode */
>>>>> +#define IOMAP_TAIL 0x06    /* tail data packed in metdata */
>>>>>
>>>>>  /*
>>>>>   * Flags for all iomap mappings:
>>>>>
>>>> .
>>>>
> .
>