On Wed, Nov 02, 2016 at 07:36:12AM -0700, Christoph Hellwig wrote: > On Tue, Nov 01, 2016 at 05:39:40PM +0100, Jan Kara wrote: > > I'd also note that having PMD-sized pages has some obvious disadvantages as > > well: > > > > 1) I'm not sure buffer head handling code will quite scale to 512 or even > > 2048 buffer_heads on a linked list referenced from a page. It may work but > > I suspect the performance will suck. > > buffer_head handling always sucks. For the iomap based bufferd write > path I plan to support a buffer_head-less mode for the block size == > PAGE_SIZE case in 4.11 latest, but if I get enough other things of my > plate in time even for 4.10. I think that's the right way to go for > THP, especially if we require the fs to allocate the whole huge page > as a single extent, similar to the DAX PMD mapping case. > > > 2) PMD-sized pages result in increased space & memory usage. > > How so? > > > 3) In ext4 we have to estimate how much metadata we may need to modify when > > allocating blocks underlying a page in the worst case (you don't seem to > > update this estimate in your patch set). With 2048 blocks underlying a page, > > each possibly in a different block group, it is a lot of metadata forcing > > us to reserve a large transaction (not sure if you'll be able to even > > reserve such large transaction with the default journal size), which again > > makes things slower. > > As said above I think we should only use huge page mappings if there is > a single underlying extent, same as in DAX to keep the complexity down. It looks like a huge limitation to me. > > 4) As you have noted some places like write_begin() still depend on 4k > > pages which creates a strange mix of places that use subpages and that use > > head pages. > > Just use the iomap bufferd I/O code and all these issues will go away. Not really. I'm looking onto iomap_write_actor(): we still calculate 'offset' and 'bytes' based on PAGE_SIZE before we even get the page. This way we limit outself to PAGE_SIZE per-iteration. -- Kirill A. Shutemov -- 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
