linux-ext4 - Re: RFC: Filesystem metadata in HIGHMEM

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Message-ID: <20230403142240.ftkywr3vn3r73yva@quack3>
Date:   Mon, 3 Apr 2023 16:22:40 +0200
From:   Jan Kara <jack@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-afs@...ts.infradead.org, ceph-devel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-nilfs@...r.kernel.org, linux-ntfs-dev@...ts.sourceforge.net,
        ntfs3@...ts.linux.dev, ocfs2-devel@....oracle.com,
        devel@...ts.orangefs.org, reiserfs-devel@...r.kernel.org,
        Evgeniy Dushistov <dushistov@...l.ru>
Subject: Re: RFC: Filesystem metadata in HIGHMEM

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@...e.com>
SUSE Labs, CR