On Mon, Oct 01, 2018 at 03:09:11PM -0700, Christoph Hellwig wrote: > It turns out the slub allocator won't always give us aligned memory, > and on some controllers this can lead to data corruption. Remove the > special slab backed fast path in xfs_buf_allocate_memory. The only > downside of this is a slight waste of memory for metadata buffers > smaller than page size. NAK. This approach creates a massive problem for 64k page size machines with sub-page size filesystem block sizes (i.e. default configurations). Every buffer will now be made up of a 64k page, even though they typically only use 4kB of that page. i.e. this blows the metadata cache footprint out by an order of magnitude and that's going to have a massive impact of system performance. Yes, we need to fix this alignment problem (that has only recently been reported for the Xen blk-front driver) but removing sub-page buffer support is not the right way to fix this. We need to: - go back to using the block device page cache and sharing pages across buffers (yuk!), or - replace the heap calls with our own aligned slabs, or - implement a generic block layer heap that guarantees storage hardware aligned sub-page buffers (as I suggested to Jens) Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
