On 2019/7/1 7:19, Darrick J. Wong wrote:
> On Sat, Jun 29, 2019 at 03:30:20PM +0800, 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
>
> Wouldn't it be easier simply to fix the meaning of IOMAP_INLINE so that
> it can be used at something other than offset 0 and length == isize?
> IOWs, make it mean "use the *inline_data pointer to read/write data
> as a direct memory access"?
I thought about that, finally I implemented this by adding a new type because:
- I checked fs.h, noticing that there are two separated flags:
FS_INLINE_DATA_FL and FS_NOTAIL_FL, I guess they are separated features, but not
sure about it...
- If we keep those restriction of inline data, maybe we can help to do sanity
check on inline data for most inline function callers additionally, since most
filesystems implement inline_data feature with restriction by default.
Anyway, I can change IOMAP_INLINE's restriction, and adjust erofs to use it if
there is no further concern on those restrictions.
>
> I also don't really like the idea of leaving the write paths
> unimplemented in core code, though I suppose as an erofs developer
> you're not likely to have a good means for testing... :/
Yes, I don't like it too, but previously I didn't add it because I'm not sure
that, recently we have potential user of IOMAP_TAIL's write path except
reiserfs, it may be out-of-{maintain,test} due to lack of user later....
>
> /me starts wondering if a better solution would be to invent iomaptestfs
> which exists solely to test all iomap code with as little other
> intelligence as possible...
Good idea, any plan on this fs? :)
Now, for erofs, as we don't support mapping hole, so I just inject code to force
covering IOMAP_HOLE path. :P
Thanks,
>
> --D
>
>> 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@xxxxxxxxxx>
>> ---
>> 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);
>> + 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:
>> --
>> 2.18.0.rc1
>>
> .
>
