Re: [PATCH 0/2] Close a hole where IOCB_NOWAIT reads could sleep

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On Sun, Jul 11, 2021 at 07:44:07PM -0600, Jens Axboe wrote:
> On 7/11/21 9:09 AM, Matthew Wilcox (Oracle) wrote:
> > I noticed a theoretical case where an IOCB_NOWAIT read could sleep:
> > 
> > filemap_get_pages
> >   filemap_get_read_batch
> >   page_cache_sync_readahead
> >     page_cache_sync_ra
> >       ondemand_readahead
> >         do_page_cache_ra
> >         page_cache_ra_unbounded
> >           gfp_t gfp_mask = readahead_gfp_mask(mapping);
> >           memalloc_nofs_save()
> >           __page_cache_alloc(gfp_mask);
> > 
> > We're in a nofs context, so we're not going to start new IO, but we might
> > wait for writeback to complete.  We generally don't want to sleep for IO,
> > particularly not for IO that isn't related to us.
> > 
> > Jens, can you run this through your test rig and see if it makes any
> > practical difference?
> 
> You bet, I'll see if I can trigger this condition and verify we're no
> longer blocking on writeback. Thanks for hacking this up.

Did you have any success yet?




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