On Thu, Dec 06, 2018 at 08:50:00AM +1100, Dave Chinner wrote: > On Wed, Dec 05, 2018 at 09:20:23AM -0800, Christoph Hellwig wrote: > > XFS currently uses a slab allocator for buffers smaller than the page > > Currently uses heap allocated memory? Or kmalloc, yeah. > I thought you were looking at using the page_frag infrastructure for > this so we didn't need a slab per filesystem for this? I did look into it, but using page_frag for buffers creates an inherent memory leak given that page_frag never reuses a freed fragement. So until all fragments in a page are free all the others bits effectively leak, which is pretty bad on the buffer cache. > I think there's a problem with this code - we can have different > sector sizes for different devices in the filesystem. e.g. the log > can have a different sector size to the data device, and this may > matter for log recovery which does a lot of single sector IO. That's fine because log recovery is one I/O at a time, and we always free the current buffer before allocating the next one, so we'd waste the memory one way or another. > Hence I think this slab really needs to be managed as a property of > the of the buftarg rather than be a global property of the > xfs_mount. in most cases the log and data devices are the same > buftarg, but we should attempt to handle this special case > correctly... We could waste another slab cache on the block device, but as said we don't really save any memory by creating a new cache, we actually waste memory due to the overhead.
