Re: [PATCH v10 10/10] xfs: enable block size larger than page size support

Darrick J. Wong
2024-07-16T22:37:02Z

On Tue, Jul 16, 2024 at 06:46:40PM +0100, Matthew Wilcox wrote:
> On Tue, Jul 16, 2024 at 10:40:16AM -0700, Darrick J. Wong wrote:
> > On Tue, Jul 16, 2024 at 04:29:05PM +0100, Matthew Wilcox wrote:
> > > On Mon, Jul 15, 2024 at 11:44:57AM +0200, Pankaj Raghav (Samsung) wrote:
> > > > +++ b/fs/xfs/xfs_super.c
> > > > @@ -1638,16 +1638,30 @@ xfs_fs_fill_super(
> > > > goto out_free_sb;
> > > > }
> > > >
> > > > - /*
> > > > - * Until this is fixed only page-sized or smaller data blocks work.
> > > > - */
> > > > if (mp->m_sb.sb_blocksize > PAGE_SIZE) {
> > > > - xfs_warn(mp,
> > > > - "File system with blocksize %d bytes. "
> > > > - "Only pagesize (%ld) or less will currently work.",
> > > > + size_t max_folio_size = mapping_max_folio_size_supported();
> > > > +
> > > > + if (!xfs_has_crc(mp)) {
> > > > + xfs_warn(mp,
> > > > +"V4 Filesystem with blocksize %d bytes. Only pagesize (%ld) or less is supported.",
> > > > mp->m_sb.sb_blocksize, PAGE_SIZE);
> > > > - error = -ENOSYS;
> > > > - goto out_free_sb;
> > > > + error = -ENOSYS;
> > > > + goto out_free_sb;
> > > > + }
> > > > +
> > > > + if (mp->m_sb.sb_blocksize > max_folio_size) {
> > > > + xfs_warn(mp,
> > > > +"block size (%u bytes) not supported; maximum folio size supported in "\
> > > > +"the page cache is (%ld bytes). Check MAX_PAGECACHE_ORDER (%d)",
> > > > + mp->m_sb.sb_blocksize, max_folio_size,
> > > > + MAX_PAGECACHE_ORDER);
> > >
> > > Again, too much message. Way too much. We shouldn't even allow block
> > > devices to be created if their block size is larger than the max supported
> > > by the page cache.
> >
> > Filesystem blocksize != block device blocksize. xfs still needs this
> > check because one can xfs_copy a 64k-fsblock xfs to a hdd with 512b
> > sectors and try to mount that on x86.
> >
> > Assuming there /is/ some fs that allows 1G blocksize, you'd then really
> > want a mount check that would prevent you from mounting that.
>
> Absolutely, we need to have an fs blocksize check in the fs (if only
> because fs fuzzers will put random values in fields and expect the system
> to not crash). But that should have nothing to do with page cache size.

I don't understand your objection -- we're setting the minimum folio
order on a file's pagecache to match the fs-wide blocksize. If the
pagecache can't possibly fulfill our fs-wide requirement, then why would
we continue the mount?

Let's pretend that MAX_PAGECACHE_ORDER is 1. The filesystem has 16k
blocks, the CPU has 4k base pages. xfs will try to set the min folio
order to 2 via mapping_set_folio_order_range. That function clamps it
to 1, so we try to cache a 16k fsblock with 8k pages. Does that
actually work?

If not, then doesn't it make more more sense to fail the mount?

--D