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SubjectRe: [patch 2/5] fs: introduce new aops and infrastructure
Dmitriy Monakhov wrote:
> Nick Piggin <npiggin@suse.de> writes:
>
>
>>Index: linux-2.6/fs/splice.c
>>===================================================================
>>--- linux-2.6.orig/fs/splice.c
>>+++ linux-2.6/fs/splice.c
>>@@ -559,7 +559,7 @@ static int pipe_to_file(struct pipe_inod
>> struct address_space *mapping = file->f_mapping;
>> unsigned int offset, this_len;
>> struct page *page;
>>- pgoff_t index;
>>+ void *fsdata;
>> int ret;
>>
>> /*
>>@@ -569,13 +569,13 @@ static int pipe_to_file(struct pipe_inod
>> if (unlikely(ret))
>> return ret;
>>
>>- index = sd->pos >> PAGE_CACHE_SHIFT;
>> offset = sd->pos & ~PAGE_CACHE_MASK;
>>
>> this_len = sd->len;
>> if (this_len + offset > PAGE_CACHE_SIZE)
>> this_len = PAGE_CACHE_SIZE - offset;
>>
>>+#if 0
>> /*
>> * Reuse buf page, if SPLICE_F_MOVE is set and we are doing a full
>> * page.
>>@@ -587,86 +587,11 @@ static int pipe_to_file(struct pipe_inod
>> * locked on successful return.
>> */
>> if (buf->ops->steal(pipe, buf))
>>- goto find_page;
>>+#endif
>
> One more note. It's looks like you just disabled all fancy zero copy logic.
> Off corse this is just rfc patchset.
> But i think where is fundamental problem with it:
> Previous logic was following:
> 1)splice code responsible for: stealing(if possible) and loking the page
> 2)prepare_write() code responsible for: do fs speciffic stuff
>
> But with new write_begin() logic all steps (grubbing, locking, preparing)
> happened internaly inside write_begin() witch doesn't even know about what
> kind of data will be copied between write_begin/write_end.
> So fancy zero copy logic is impossible :(

Check linux-mm: zero-copy splice is broken anyway, and AFAIKS it cannot
really be fixed to work with the current prepare_write.

> I think this can be solved somehow, but i dont know yet, how can this be done
> without implementing it inside begin_write().

Actually we could do it with begin_write. All we need to do is set a
flag to say that *pagep contains a page that we can use, with a copy of
the write data on it.

The filesystem would then be able to insert that page into the pagecache
*if* it could handle such an operation.

OTOH, is splice stealing support really important? I guess it could be a
nice win for a small niche of workloads, and we probably don't want to
exclude it by design...

--
SUSE Labs, Novell Inc.
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