On 08/04/2011 10:44 AM, Mingming Cao wrote:
On Thu, 2011-08-04 at 11:25 -0400, Ted Ts'o wrote:
On Thu, Aug 04, 2011 at 12:22:58AM -0700, Allison Henderson wrote:
Oh, I think we do avoid calling the unmap for this last condition
though. The first and last page offsets are calculated earlier for
calling truncate_inode_pages_range to release all the pages in the
hole. The idea is that everything from first_page_offset to
last_page_offset covers all the page aligned pages in the hole. So
then if offset and length are aligned, we basically end up with
first_page_offset = offset and last_page_offset = offset + length,
and the page_len will turn out to be zero. Right math? Maybe we
can add some comments or something to help clarify.
Yeah, sorry, I wasn't clear enough about the condition. Consider the
situation where we punch the region:
4092 -- 8197
In the previous section of code, we would zero out the byte ranges
4092--4095 and 8192--8197. What's left is a completely page-aligned
range, which would have already been taken care of already. But since
we're calculating based on offsets, I believe there will be an
unnecessary call to ext4_unmap_page_range().
Yep, for the default 4k block size, if the offset is not block aligned,
with the patch we could end of unnecessary unamp_page_range.
BTW, the name ext4_unmap_page_range() is a bit confusing; maybe we
should rename it to ext4_unmap_partial_page_buffers()?
The new name sounds better. It should only called for punch hole in the
range (blocksize != pagesize) and (offset is block aligned) and (offset
is not page aligned)
I know you were copying from the ext4_block_zero_page_range() function
and its calling sequence (but in my opinion that function wasn't named
well and the comments in that code aren't good either).
I also wonder why we can't fold the functionality found in
ext4_unmap_page_range() into ext4_block_zero_page_range(). Did you
look into that option?
ext4_block_zero_page_range() also called from ext4 truncate code path,
which only zero out within a block, but do not need to handle the
partial page unmap. There are two logical steps need by punch hole, one
is to zero out the non-block-aligned portion(like truncate), second is
to unmap_partial_page_buffers(). It seems cleaner to separate the two
logical steps out from the code simplify point of view.
Regards,
Mingming
Yeah looking at them again, I think I like the simpler v3. V2 does both
operations in one loop, but I think it's cleaner to keep them separate.
Allison Henderson
Regards,
- Ted
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