From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kara Subject: Re: RFC: Filesystem metadata in HIGHMEM Date: Mon, 3 Apr 2023 16:22:40 +0200 Message-ID: <20230403142240.ftkywr3vn3r73yva@quack3> References: Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1680531761; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=R46s3dzmAWjax7iXZKikFYCLr1xSbo6uFhX0zzjg4pU=; b=XViU3+ETjmX+Zv1i3Q4VVTn7Ny+KHI+pcGRnoPPT3r61aXIzJBxPYzNkNl9++TJ9aBjV6M pj5xzmCEcljzJ3VrWr7zg8WDaJErfGUilAp+755lKhp6uAWpsjuSCcyiyy9snmSveJO8Pa dYp4hCNxKq+rm1Sy8iI/zHvhEXqBk/Y= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1680531761; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=R46s3dzmAWjax7iXZKikFYCLr1xSbo6uFhX0zzjg4pU=; b=U1FjLpLfehFJEz9putWhis2w9XqHu0U3h3oaiL98jCSHliu31HtoSv01Qgpvds/nc1S0hp frS9z8ZPoJjwAoCw== Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Matthew Wilcox Cc: linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-afs-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, ceph-devel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-nfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-nilfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-ntfs-dev-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, ntfs3-cunTk1MwBs/YUNznpcFYbw@public.gmane.org, ocfs2-devel-N0ozoZBvEnrZJqsBc5GL+g@public.gmane.org, devel-urhm25FSIHzptaDgeN77Px2eb7JE58TQ@public.gmane.org, reiserfs-devel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Evgeniy Dushistov 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 SUSE Labs, CR