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Date: Mon, 26 Feb 2024 14:40:42 +0000
From: Matthew Wilcox <willy@infradead.org>
To: "Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>
Cc: linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, david@fromorbit.com,
	chandan.babu@oracle.com, akpm@linux-foundation.org,
	mcgrof@kernel.org, ziy@nvidia.com, hare@suse.de, djwong@kernel.org,
	gost.dev@samsung.com, linux-mm@kvack.org,
	Pankaj Raghav <p.raghav@samsung.com>
Subject: Re: [PATCH 03/13] filemap: align the index to mapping_min_order in
 the page cache
Message-ID: <Zdyi6lFDAHXi8GPz@casper.infradead.org>
References: <20240226094936.2677493-1-kernel@pankajraghav.com>
 <20240226094936.2677493-4-kernel@pankajraghav.com>
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In-Reply-To: <20240226094936.2677493-4-kernel@pankajraghav.com>

On Mon, Feb 26, 2024 at 10:49:26AM +0100, Pankaj Raghav (Samsung) wrote:
> From: Luis Chamberlain <mcgrof@kernel.org>
> 
> Supporting mapping_min_order implies that we guarantee each folio in the
> page cache has at least an order of mapping_min_order. So when adding new
> folios to the page cache we must 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.

This seems like a remarkably complicated way of achieving:

diff --git a/mm/filemap.c b/mm/filemap.c
index 5603ced05fb7..36105dad4440 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2427,9 +2427,11 @@ 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)
 {
+	pgoff_t index;
+	unsigned int min_order;
 	struct folio *folio;
 	int error;
 
@@ -2451,6 +2453,8 @@ static int filemap_create_folio(struct file *file,
 	 * well to keep locking rules simple.
 	 */
 	filemap_invalidate_lock_shared(mapping);
+	min_order = mapping_min_folio_order(mapping);
+	index = (pos >> (min_order + PAGE_SHIFT)) << min_order;
 	error = filemap_add_folio(mapping, folio, index,
 			mapping_gfp_constraint(mapping, GFP_KERNEL));
 	if (error == -EEXIST)
@@ -2511,8 +2515,7 @@ 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;