Re: [PATCH] mm: avoid blocking lock_page() in kcompactd

[Matthew Wilcox]

On Thu, Feb 13, 2020 at 06:08:24PM +0100, Michal Hocko wrote:
> On Thu 13-02-20 08:46:07, Matthew Wilcox wrote:
> > On Thu, Feb 13, 2020 at 08:48:47AM +0100, Michal Hocko wrote:
> > > Can we pursue on this please? An explicit NOFS scope annotation with a
> > > reference to compaction potentially locking up on pages in the readahead
> > > would be a great start.
> > 
> > How about this (on top of the current readahead series):
> > 
> > diff --git a/mm/readahead.c b/mm/readahead.c
> > index 29ca25c8f01e..32fd32b913da 100644
> > --- a/mm/readahead.c
> > +++ b/mm/readahead.c
> > @@ -160,6 +160,16 @@ unsigned long page_cache_readahead_limit(struct address_space *mapping,
> >  		.nr_pages = 0,
> >  	};
> >  
> > +	/*
> > +	 * Partway through the readahead operation, we will have added
> > +	 * locked pages to the page cache, but will not yet have submitted
> > +	 * them for I/O.  Adding another page may need to allocate
> > +	 * memory, which can trigger memory migration.	Telling the VM
> 
> I would go with s@migration@compaction@ because it would make it more
> obvious that this is about high order allocations.

Perhaps even just 'reclaim' -- it's about compaction today, but tomorrow's
VM might try to reclaim these pages too.  They are on the LRU, after all.

So I currently have:

        /*
         * Partway through the readahead operation, we will have added
         * locked pages to the page cache, but will not yet have submitted
         * them for I/O.  Adding another page may need to allocate memory,
         * which can trigger memory reclaim.  Telling the VM we're in
         * the middle of a filesystem operation will cause it to not
         * touch file-backed pages, preventing a deadlock.  Most (all?)
         * filesystems already specify __GFP_NOFS in their mapping's
         * gfp_mask, but let's be explicit here.
         */

Thanks!