linux-kernel - Re: [PATCH v4 03/11] filemap: allocate mapping_min

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Message-ID: <20240426151243.GD360919@frogsfrogsfrogs>
Date: Fri, 26 Apr 2024 08:12:43 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: willy@...radead.org, brauner@...nel.org, david@...morbit.com,
	chandan.babu@...cle.com, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, hare@...e.de,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-xfs@...r.kernel.org, mcgrof@...nel.org, gost.dev@...sung.com,
	p.raghav@...sung.com
Subject: Re: [PATCH v4 03/11] filemap: allocate mapping_min_order folios in
 the page cache

On Thu, Apr 25, 2024 at 01:37:38PM +0200, Pankaj Raghav (Samsung) wrote:
> From: Luis Chamberlain <mcgrof@...nel.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@...nel.org>

--D

> Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> Co-developed-by: Pankaj Raghav <p.raghav@...sung.com>
> Signed-off-by: Pankaj Raghav <p.raghav@...sung.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
> 
>