Re: [PATCH] iomap: fix race when reading in all bytes of a folio

[Christoph Hellwig <hch@infradead.org>]

On Fri, Oct 24, 2025 at 02:50:08PM -0700, Joanne Koong wrote:
> There is a race where if all bytes in a folio need to get read in and
> the filesystem finishes reading the bytes in before the call to
> iomap_read_end(), then bytes_accounted in iomap_read_end() will be 0 and
> the following "ifs->read_bytes_pending -= bytes_accounting" will also be
> 0 which will trigger an extra folio_end_read() call. This extra
> folio_end_read() unlocks the folio for the 2nd time, which sets the lock
> bit on the folio, resulting in a permanent lockup.
> 
> Fix this by returning from iomap_read_end() early if all bytes are read
> in by the filesystem.
> 
> Additionally, add some comments to clarify how this accounting logic works.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> Fixes: 51311f045375 ("iomap: track pending read bytes more optimally")
> Reported-by: Brian Foster <bfoster@redhat.com>

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

> This is a fix for commit 51311f045375 in the 'vfs-6.19.iomap' branch. It
> would be great if this could get folded up into that original commit, if it's
> not too logistically messy to do so.

Agreed!