From: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
To: Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
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 <dushistov-JGs/UdohzUI@public.gmane.org>
Subject: Re: RFC: Filesystem metadata in HIGHMEM
Date: Mon, 3 Apr 2023 16:22:40 +0200 [thread overview]
Message-ID: <20230403142240.ftkywr3vn3r73yva@quack3> (raw)
In-Reply-To: <ZBCJ11qT8AWGA9y8-FZi0V3Vbi30CUdFEqe4BF2D2FQJk+8+b@public.gmane.org>
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-IBi9RG/b67k@public.gmane.org>
SUSE Labs, CR
prev parent reply other threads:[~2023-04-03 14:22 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-14 14:51 RFC: Filesystem metadata in HIGHMEM Matthew Wilcox
[not found] ` <ZBCJ11qT8AWGA9y8-FZi0V3Vbi30CUdFEqe4BF2D2FQJk+8+b@public.gmane.org>
2023-04-03 14:22 ` Jan Kara [this message]
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