Re: next-20250721 arm64 16K and 64K page size WARNING fs fuse file.c at fuse_iomap_writeback_range

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jul 28, 2025 at 10:55:42AM -0700, Joanne Koong wrote:
> On Mon, Jul 28, 2025 at 10:34 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Fri, Jul 25, 2025 at 06:16:15PM -0700, Joanne Koong wrote:
> > > > > > > Also, I just noticed that apparently the blocksize can change
> > > > > > > dynamically for an inode in fuse through getattr replies from the
> > > > > > > server (see fuse_change_attributes_common()). This is a problem since
> > > > > > > the iomap uses inode->i_blkbits for reading/writing to the bitmap. I
> > > > > > > think we will have to cache the inode blkbits in the iomap_folio_state
> > > > > > > struct unfortunately :( I'll think about this some more and send out a
> > > > > > > patch for this.
> >
> > Does this actually happen in practice, once you've started _using_ the
> > block device?  Rather than all this complicated stuff to invalidate the

For most block device filesystems?  No.  And as far as I can tell, none
of the filesystems actually support changing i_blkbits on the fly; I
think only block devices can do that:

$ git grep 'i_blkbits\s=\s'
block/bdev.c:150:       BD_INODE(bdev)->i_blkbits = blksize_bits(bsize);
block/bdev.c:209:               inode->i_blkbits = blksize_bits(size);
fs/ceph/inode.c:81:     inode->i_blkbits = CEPH_FSCRYPT_BLOCK_SHIFT;
fs/ceph/inode.c:1071:           inode->i_blkbits = CEPH_FSCRYPT_BLOCK_SHIFT;
fs/ceph/inode.c:1076:           inode->i_blkbits = CEPH_BLOCK_SHIFT;
fs/ceph/inode.c:1180:           inode->i_blkbits = PAGE_SHIFT;
fs/direct-io.c:612:     unsigned int i_blkbits = sdio->blkbits + sdio->blkfactor;
fs/direct-io.c:908:     const unsigned i_blkbits = blkbits + sdio->blkfactor;
fs/direct-io.c:1110:    unsigned i_blkbits = READ_ONCE(inode->i_blkbits);
fs/erofs/fscache.c:527: inode->i_blkbits = EROFS_SB(sb)->blkszbits;
fs/fuse/file_iomap.c:2327:      inode->i_blkbits = new_blkbits;
fs/fuse/inode.c:304:            inode->i_blkbits = new_blkbits;
fs/inode.c:234: inode->i_blkbits = sb->s_blocksize_bits;
fs/libfs.c:1761:        inode->i_blkbits = PAGE_SHIFT;
fs/ocfs2/aops.c:2123:   unsigned int i_blkbits = inode->i_sb->s_blocksize_bits;
fs/orangefs/orangefs-utils.c:320:                       inode->i_blkbits = ffs(new_op->downcall.resp.getattr.
fs/smb/client/cifsfs.c:411:     cifs_inode->netfs.inode.i_blkbits = 14;  /* 2**14 = CIFS_MAX_MSGSIZE */
fs/stack.c:72:  dest->i_blkbits = src->i_blkbits;
fs/vboxsf/utils.c:123:  inode->i_blkbits = 12;

> > page cache based on the fuse server telling us something, maybe just
> > declare the server to be misbehaving and shut the whole filesystem down?
> >
> 
> I don't think this case is likely at all but I guess one scenario
> where the server might want to change the block size midway through is
> if they send the data to some network filesystem on the backend and if
> that backend shuts down or is at full capacity for whatever reason and
> they need to migrate to another backend that uses a different block
> size then I guess this would be useful for that.

Maybe, but it would still be pretty extraordinary to change the block
size on an open file -- any program that tries to do its IO in blocks
(i.e. not a byte stream) has already stat'd the file and will be very
confused.

> fuse currently does allow the block size to be changed dynamically so
> I'm not sure if we can change that behavior without breaking backwards
> compatibility.

<nod> For fuse+iomap I'm not going to allow it initially if there's
anything in the pagecache ... but I could be talked into it.

--D