Re: [PATCH v3 5/9] xfs: refuse a direct allocation whose block reservation would wrap

[Christoph Hellwig <hch@lst.de>]

On Thu, Oct 15, 2026 at 01:08:04PM +0900, Daejun Park via B4 Relay wrote:
> From: Daejun Park <daejun7.park@samsung.com>
> 
> xfs_iomap_write_direct() maps one extent, but it reserves blocks for the
> whole count it is given, in an unsigned int. A count of about 2^32
> blocks or more wraps the reservation. If it wraps to a few blocks, the
> allocation uses more blocks than the transaction reserved, and
> xfs_trans_mod_sb() shuts the filesystem down. If it wraps to nearly 2^32
> blocks, the allocation fails with -ENOSPC.
> 
> Direct I/O, DAX and buffered writes with an extent size hint never ask
> for that much, as xfs_direct_write_iomap_begin() limits them to
> 1024 pages to keep the count below 32 bits. xfs_fs_map_blocks() has no
> such limit: it asks for everything from the start of a hole to the end
> of the range of a pNFS layout, which a client chooses, and an RW
> LAYOUTGET at offset 0 of an empty file for 16 TiB does it.
> 
> Fail such a count with -ENOSPC before anything is reserved, also on a
> filesystem with that much free space, as the transaction cannot take it.
> A count that fits is reserved in full, as before, so a request for more
> blocks than are free still fails before anything is allocated.

I think the limit needs to be lower, as xfs_bmapi_write can't hand
out such large limits.  We'll also need to make sure pnfs can
request multiple maps.