linux-kernel - Re: [PATCH v9 04/10] mm: split a folio in minimum folio order chunks

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Message-ID: <20240709110423.wdteahoeufrt22jk@quentin>
Date: Tue, 9 Jul 2024 11:04:23 +0000
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Zi Yan <ziy@...dia.com>
Cc: david@...morbit.com, willy@...radead.org, chandan.babu@...cle.com,
	djwong@...nel.org, brauner@...nel.org, akpm@...ux-foundation.org,
	yang@...amperecomputing.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, john.g.garry@...cle.com,
	linux-fsdevel@...r.kernel.org, hare@...e.de, p.raghav@...sung.com,
	mcgrof@...nel.org, gost.dev@...sung.com, cl@...amperecomputing.com,
	linux-xfs@...r.kernel.org, hch@....de
Subject: Re: [PATCH v9 04/10] mm: split a folio in minimum folio order chunks

> 
> This should be
> 
> 		if (!folio->mapping) {
> 			if (folio_test_pmd_mappable(folio))
> 				count_vm_event(THP_SPLIT_PAGE_FAILED);
> 			return -EBUSY;
> 		}
> 
> Otherwise, a non PMD mappable folio with no mapping will fall through
> and cause NULL pointer dereference in mapping_min_folio_order().

Ah, of course. I thought I was being "smart" here to avoid another
nesting. Instead of triple nested ifs, I guess this is better:

int split_folio_to_list(struct folio *folio, struct list_head *list)
{
       unsigned int min_order = 0;

       if (folio_test_anon(folio))
               goto out;

       if (!folio->mapping) {
               if (folio_test_pmd_mappable(folio))
                       count_vm_event(THP_SPLIT_PAGE_FAILED);
               return -EBUSY;
       }

       min_order = mapping_min_folio_order(folio->mapping);
out:
       return split_huge_page_to_list_to_order(&folio->page, list,
                                                       min_order);
}

Let me know what you think!

> 
> > +		min_order = mapping_min_folio_order(folio->mapping);
> > +	}
> > +
> > +	return split_huge_page_to_list_to_order(&folio->page, list, min_order);
> > +}
> > +
> >  void __folio_undo_large_rmappable(struct folio *folio)
> >  {
> >  	struct deferred_split *ds_queue;
> 
> 
> -- 
> Best Regards,
> Yan, Zi
>