Re: [PATCH] mm/filemap: set folio->mapping to NULL before xas_store()

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 26, 2024 at 02:05:33PM -0700, Andrew Morton wrote:
> On Sat, 23 Mar 2024 06:04:54 +0900 Soma Nakata <soma.nakata01@gmail.com> wrote:
> > Functions such as __filemap_get_folio() check the truncation of
> > folios based on the mapping field. Therefore setting this field to NULL
> > earlier prevents unnecessary operations on already removed folios.
> > 
> > ...
> >
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -139,11 +139,12 @@ static void page_cache_delete(struct address_space *mapping,
> >  
> >  	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> >  
> > +	folio->mapping = NULL;
> > +	/* Leave page->index set: truncation lookup relies upon it */
> > +
> >  	xas_store(&xas, shadow);
> >  	xas_init_marks(&xas);
> >  
> > -	folio->mapping = NULL;
> > -	/* Leave page->index set: truncation lookup relies upon it */
> >  	mapping->nrpages -= nr;
> >  }
> 
> Seems at least harmless, but I wonder if it can really make any
> difference.  Don't readers of folio->mapping lock the folio first?

I can't think of anywhere that doesn't ... most of the places that check
folio->mapping have "goto unlock" as the very next line.  I don't think
this patch accomplishes anything.