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Date: Sun, 28 Apr 2024 20:59:15 +0000
From: "Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: willy@infradead.org, brauner@kernel.org, david@fromorbit.com,
	chandan.babu@oracle.com, akpm@linux-foundation.org,
	linux-fsdevel@vger.kernel.org, hare@suse.de,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-xfs@vger.kernel.org, mcgrof@kernel.org, gost.dev@samsung.com,
	p.raghav@samsung.com
Subject: Re: [PATCH v4 03/11] filemap: allocate mapping_min_order folios in
 the page cache
Message-ID: <20240428205915.2iocwkcf3edc5y2k@quentin>
References: <20240425113746.335530-1-kernel@pankajraghav.com>
 <20240425113746.335530-4-kernel@pankajraghav.com>
 <20240426151243.GD360919@frogsfrogsfrogs>
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In-Reply-To: <20240426151243.GD360919@frogsfrogsfrogs>

On Fri, Apr 26, 2024 at 08:12:43AM -0700, Darrick J. Wong wrote:
> On Thu, Apr 25, 2024 at 01:37:38PM +0200, Pankaj Raghav (Samsung) wrote:
> > From: Luis Chamberlain <mcgrof@kernel.org>
> > 
> > filemap_create_folio() and do_read_cache_folio() were always allocating
> > folio of order 0. __filemap_get_folio was trying to allocate higher
> > order folios when fgp_flags had higher order hint set but it will default
> > to order 0 folio if higher order memory allocation fails.
> > 
> > Supporting mapping_min_order implies that we guarantee each folio in the
> > page cache has at least an order of mapping_min_order. When adding new
> > folios to the page cache we must also ensure the index used is aligned to
> > the mapping_min_order as the page cache requires the index to be aligned
> > to the order of the folio.
> 
> If we cannot find a folio of at least min_order size, what error is sent
> back?
> 
> If the answer is "the same error that you get if we cannot allocate a
> base page today (aka ENOMEM)", then I think I understand this enough to
> say
> 
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Yes. We will get a ENOMEM if we cannot allocate min_order size folio. :)
Thanks!
> 
> --D
> 
> > Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> > Co-developed-by: Pankaj Raghav <p.raghav@samsung.com>
> > Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> > ---
> >  mm/filemap.c | 24 +++++++++++++++++-------
> >  1 file changed, 17 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 30de18c4fd28..f0c0cfbbd134 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -858,6 +858,8 @@ noinline int __filemap_add_folio(struct address_space *mapping,
> >  
> >  	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> >  	VM_BUG_ON_FOLIO(folio_test_swapbacked(folio), folio);
> > +	VM_BUG_ON_FOLIO(folio_order(folio) < mapping_min_folio_order(mapping),
> > +			folio);
> >  	mapping_set_update(&xas, mapping);
> >  
> >  	if (!huge) {
> > @@ -1895,8 +1897,10 @@ 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 min_order = mapping_min_folio_order(mapping);
> > +		unsigned int order = max(min_order, FGF_GET_ORDER(fgp_flags));
> >  		int err;
> > +		index = mapping_align_start_index(mapping, index);
> >  
> >  		if ((fgp_flags & FGP_WRITE) && mapping_can_writeback(mapping))
> >  			gfp |= __GFP_WRITE;
> > @@ -1936,7 +1940,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
> >  				break;
> >  			folio_put(folio);
> >  			folio = NULL;
> > -		} while (order-- > 0);
> > +		} while (order-- > min_order);
> >  
> >  		if (err == -EEXIST)
> >  			goto repeat;
> > @@ -2425,13 +2429,16 @@ static int filemap_update_page(struct kiocb *iocb,
> >  }
> >  
> >  static int filemap_create_folio(struct file *file,
> > -		struct address_space *mapping, pgoff_t index,
> > +		struct address_space *mapping, loff_t pos,
> >  		struct folio_batch *fbatch)
> >  {
> >  	struct folio *folio;
> >  	int error;
> > +	unsigned int min_order = mapping_min_folio_order(mapping);
> > +	pgoff_t index;
> >  
> > -	folio = filemap_alloc_folio(mapping_gfp_mask(mapping), 0);
> > +	folio = filemap_alloc_folio(mapping_gfp_mask(mapping),
> > +				    min_order);
> >  	if (!folio)
> >  		return -ENOMEM;
> >  
> > @@ -2449,6 +2456,8 @@ static int filemap_create_folio(struct file *file,
> >  	 * well to keep locking rules simple.
> >  	 */
> >  	filemap_invalidate_lock_shared(mapping);
> > +	/* index in PAGE units but aligned to min_order number of pages. */
> > +	index = (pos >> (PAGE_SHIFT + min_order)) << min_order;
> >  	error = filemap_add_folio(mapping, folio, index,
> >  			mapping_gfp_constraint(mapping, GFP_KERNEL));
> >  	if (error == -EEXIST)
> > @@ -2509,8 +2518,7 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
> >  	if (!folio_batch_count(fbatch)) {
> >  		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
> >  			return -EAGAIN;
> > -		err = filemap_create_folio(filp, mapping,
> > -				iocb->ki_pos >> PAGE_SHIFT, fbatch);
> > +		err = filemap_create_folio(filp, mapping, iocb->ki_pos, fbatch);
> >  		if (err == AOP_TRUNCATED_PAGE)
> >  			goto retry;
> >  		return err;
> > @@ -3708,9 +3716,11 @@ static struct folio *do_read_cache_folio(struct address_space *mapping,
> >  repeat:
> >  	folio = filemap_get_folio(mapping, index);
> >  	if (IS_ERR(folio)) {
> > -		folio = filemap_alloc_folio(gfp, 0);
> > +		folio = filemap_alloc_folio(gfp,
> > +					    mapping_min_folio_order(mapping));
> >  		if (!folio)
> >  			return ERR_PTR(-ENOMEM);
> > +		index = mapping_align_start_index(mapping, index);
> >  		err = filemap_add_folio(mapping, folio, index, gfp);
> >  		if (unlikely(err)) {
> >  			folio_put(folio);
> > -- 
> > 2.34.1
> > 
> >