Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Reclamation interactions with RCU

Matthew Wilcox
2024-02-29T04:25:04Z

On Wed, Feb 28, 2024 at 11:17:33PM -0500, Kent Overstreet wrote:
> On Wed, Feb 28, 2024 at 07:37:58PM +0000, Matthew Wilcox wrote:
> > Perhaps broaden this slightly. On the THP Cabal call we just had a
> > conversation about the requirements on filesystems in the writeback
> > path. We currently tell filesystem authors that the entire writeback
> > path must avoid allocating memory in order to prevent deadlock (or use
> > GFP_MEMALLOC). Is this appropriate? It's a lot of work to assure that
> > writing pagecache back will not allocate memory in, eg, the network stack,
> > the device driver, and any other layers the write must traverse.
>
> Why would you not simply mark the writeback path with
> memalloc_nofs_save()?

It's not about preventing recursion, it's about guaranteeing forward
progres. If you can't allocate a bio, you can't clean memory.