Re: [PATCH 04/13] filemap: use mapping_min_order while allocating folios

[Pankaj Raghav (Samsung)]

On Mon, Feb 26, 2024 at 02:47:33PM +0000, Matthew Wilcox wrote:
> On Mon, Feb 26, 2024 at 10:49:27AM +0100, Pankaj Raghav (Samsung) wrote:
> > Add some additional VM_BUG_ON() in page_cache_delete[batch] and
> > __filemap_add_folio to catch errors where we delete or add folios that
> > has order less than min_order.
> 
> I don't understand why we need these checks in the deletion path.  The
> add path, yes, absolutely.  But the delete path?
I think we initially added it to check if some split happened which
might mess up the page cache with min order support. But I think it is
not super critical anymore because of the changes in the split_folio
path. I will remove the checks.

> 
> > @@ -896,6 +900,8 @@ noinline int __filemap_add_folio(struct address_space *mapping,
> >  			}
> >  		}
> >  
> > +		VM_BUG_ON_FOLIO(folio_order(folio) < mapping_min_folio_order(mapping),
> > +				folio);
> 
> But I don't understand why you put it here, while we're holding the
> xa_lock.  That seems designed to cause maximum disruption.  Why not put
> it at the beginning of the function with all the other VM_BUG_ON_FOLIO?

Yeah. That makes sense as the folio itself is not changing.

> 
> > @@ -1847,6 +1853,9 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> >  		fgf_t fgp_flags, gfp_t gfp)
> >  {
> >  	struct folio *folio;
> > +	unsigned int min_order = mapping_min_folio_order(mapping);
> > +
> > +	index = mapping_align_start_index(mapping, index);
> 
> I would not do this here.
> 
> >  repeat:
> >  	folio = filemap_get_entry(mapping, index);
> > @@ -1886,7 +1895,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> >  		folio_wait_stable(folio);
> >  no_page:
> >  	if (!folio && (fgp_flags & FGP_CREAT)) {
> > -		unsigned order = FGF_GET_ORDER(fgp_flags);
> > +		unsigned int order = max(min_order, FGF_GET_ORDER(fgp_flags));
> >  		int err;
> 
> Put it here instead.
> 
> >  		if ((fgp_flags & FGP_WRITE) && mapping_can_writeback(mapping))
> > @@ -1912,8 +1921,13 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> >  			gfp_t alloc_gfp = gfp;
> >  
> >  			err = -ENOMEM;
> > +			if (order < min_order)
> > +				order = min_order;
> >  			if (order > 0)
> >  				alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;
> > +
> > +			VM_BUG_ON(index & ((1UL << order) - 1));
> 
> Then you don't need this BUG_ON because it's obvious you just did it.
> And the one in filemap_add_folio() would catch it anyway.

I agree. I will change it in the next revision.