On Tue 14-03-23 14:51:03, Matthew Wilcox wrote: > TLDR: I think we should rip out support for fs metadata in highmem > > We want to support filesystems on devices with LBA size > PAGE_SIZE. > That's subtly different and slightly harder than fsblk size > PAGE_SIZE. > We can use large folios to read the blocks into, but reading/writing > the data in those folios is harder if it's in highmem. The kmap family > of functions can only map a single page at a time (and changing that > is hard). We could vmap, but that's slow and can't be used from atomic > context. Working a single page at a time can be tricky (eg consider an > ext2 directory entry that spans a page boundary). > > Many filesystems do not support having their metadata in highmem. > ext4 doesn't. xfs doesn't. f2fs doesn't. afs, ceph, ext2, hfs, > minix, nfs, nilfs2, ntfs, ntfs3, ocfs2, orangefs, qnx6, reiserfs, sysv > and ufs do. > > Originally, ext2 directories in the page cache were done by Al Viro > in 2001. At that time, the important use-case was machines with tens of > gigabytes of highmem and ~800MB of lowmem. Since then, the x86 systems > have gone to 64-bit and the only real uses for highmem are cheap systems > with ~8GB of memory total and 2-4GB of lowmem. These systems really > don't need to keep directories in highmem; using highmem for file & > anon memory is enough to keep the system in balance. > > So let's just rip out the ability to keep directories (and other fs > metadata) in highmem. Many filesystems already don't support this, > and it makes supporting LBA size > PAGE_SIZE hard. > > I'll turn this into an LSFMM topic if we don't reach resolution on the > mailing list, but I'm optimistic that everybody will just agree with > me ;-) FWIW I won't object for the local filesystems I know about ;). But you mention some networking filesystems above like NFS, AFS, orangefs - how are they related to the LBA size problem you mention and what exactly you want to get rid of there? FWIW I can imagine some 32-bit system (possibly diskless) that uses NFS and that would benefit in caching stuff in highmem... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
