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 > Regards, > > - Ted > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
