Re: error writing primary super block on zoned btrfs

[Christoph Hellwig <hch@lst.de>]

On Mon, Jul 18, 2022 at 01:28:52PM +0100, Matthew Wilcox wrote:
> A "value" entry in the block device's i_pages will be a shadow entry --
> that is, the page has reached the end of the LRU list and been discarded,
> so we made a note that we would have liked to keep it in the LRU list,
> but we didn't have enough memory in the system to do so.  That helps
> us put it back in the right position in the LRU list when it gets
> brought back in from disc.
> 
> I'd sugegst something else has gone wrong, maybe the refcount should
> have been kept elevated to prevent the superblock from being paged out.
> I find it hard to believe that we can be so low on memory that we need
> to page out a superblock to make room for some other memory allocation.
> 
> (Although maybe if you have millions of unusued filesystems mounted ...?)

This is a freshly booted VM running the test on the only non-root
disk file system.  So yeah, there must be a logic error somewhere
in the use of the page cache.