On Wed, Nov 27, 2024 at 11:17:37AM +0530, Bharata B Rao wrote: > In order to experiment using large folios for block devices read/write > operations, expose an ioctl that userspace can selectively use on the > raw block devices. > > For the write path, this forces iomap layer to provision large > folios (via iomap_file_buffered_write()). Well, unless CONFIG_BUFFER_HEAD is disabled, the block device uses the buffer head based write path, which currently doesn't fully support large folios (although there is series out to do so on fsdevel right now), so I don't think this will fully work. But the more important problem, and the reason why we don't use the non-buffer_head path by default is that the block device mapping is reused by a lot of file systems, which are not aware of large folios, and will get utterly confused. So if we want to do anything smart on the block device mapping, we'll have to ensure we're back to state compatible with these file systems before calling into their mount code, and stick to the old code while file systems are mounted. Of course the real question is: why do you care about buffered I/O performance on the block device node?
