How to debug stuck read?

[Matthew Wilcox]

On Wed, Feb 02, 2022 at 07:15:14PM +0200, Dāvis Mosāns wrote:
> I have a corrupted file on BTRFS which has CoW disabled thus no
> checksum. Trying to read this file causes the process to get stuck
> forever. It doesn't return EIO.
> 
> How can I find out why it gets stuck?

> $ cat /proc/3449/stack | ./scripts/decode_stacktrace.sh vmlinux
> folio_wait_bit_common (mm/filemap.c:1314)
> filemap_get_pages (mm/filemap.c:2622)
> filemap_read (mm/filemap.c:2676)
> new_sync_read (fs/read_write.c:401 (discriminator 1))

folio_wait_bit_common() is where it waits for the page to be unlocked.
Probably the problem is that btrfs isn't unlocking the page on
seeing the error, so you don't get the -EIO returned?