linux-kernel - [PATCH] fs/isofs: Replace kmap() with kmap_local

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Message-Id: <20220731190101.7928-1-fmdefrancesco@gmail.com>
Date:   Sun, 31 Jul 2022 21:01:01 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Jan Kara <jack@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Pali Rohár <pali@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <songmuchun@...edance.com>,
        Ira Weiny <ira.weiny@...el.com>, linux-kernel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Subject: [PATCH] fs/isofs: Replace kmap() with kmap_local_page()

The use of kmap() is being deprecated in favor of kmap_local_page().

There are two main problems with kmap(): (1) It comes with an overhead as
mapping space is restricted and protected by a global lock for
synchronization and (2) it also requires global TLB invalidation when the
kmap’s pool wraps and it might block when the mapping space is fully
utilized until a slot becomes available.

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).
Tasks can be preempted and, when scheduled to run again, the kernel
virtual addresses are restored and still valid. It is faster than kmap()
in kernels with HIGHMEM enabled.

Since kmap_local_page() can be safely used in compress.c, it should be
called everywhere instead of kmap().

Therefore, replace kmap() with kmap_local_page() in compress.c. Where it
is needed, use memzero_page() instead of open coding kmap_local_page()
plus memset() to fill the pages with zeros. Delete the redundant
flush_dcache_page() in the two call sites of memzero_page().

Tested with mkisofs on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel
with HIGHMEM64GB enabled.

Cc: Jan Kara <jack@...e.cz>
Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---

Many thanks to Jan Kara for the comments and suggestions provided as
reply to my previous RFC.[1] Furthermore, I want to thank Ira Weiny for
the advice he provided privately, especially about how to use mkisofs to
test that this patch works properly.

[1] https://lore.kernel.org/lkml/20220726145024.rryvw7ot7j2c6tqv@quack3/ 

 fs/isofs/compress.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/fs/isofs/compress.c b/fs/isofs/compress.c
index 95a19f25d61c..f17754484a75 100644
--- a/fs/isofs/compress.c
+++ b/fs/isofs/compress.c
@@ -67,8 +67,7 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
 		for ( i = 0 ; i < pcount ; i++ ) {
 			if (!pages[i])
 				continue;
-			memset(page_address(pages[i]), 0, PAGE_SIZE);
-			flush_dcache_page(pages[i]);
+			memzero_page(pages[i], 0, PAGE_SIZE);
 			SetPageUptodate(pages[i]);
 		}
 		return ((loff_t)pcount) << PAGE_SHIFT;
@@ -120,7 +119,7 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
 	       zerr != Z_STREAM_END) {
 		if (!stream.avail_out) {
 			if (pages[curpage]) {
-				stream.next_out = page_address(pages[curpage])
+				stream.next_out = kmap_local_page(pages[curpage])
 						+ poffset;
 				stream.avail_out = PAGE_SIZE - poffset;
 				poffset = 0;
@@ -176,6 +175,10 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
 				flush_dcache_page(pages[curpage]);
 				SetPageUptodate(pages[curpage]);
 			}
+			if (stream.next_out != (char *)zisofs_sink_page) {
+				kunmap_local(stream.next_out);
+				stream.next_out = NULL;
+			}
 			curpage++;
 		}
 		if (!stream.avail_in)
@@ -183,6 +186,8 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
 	}
 inflate_out:
 	zlib_inflateEnd(&stream);
+	if (stream.next_out && stream.next_out != (char *)zisofs_sink_page)
+		kunmap_local(stream.next_out);
 
 z_eio:
 	mutex_unlock(&zisofs_zlib_lock);
@@ -283,9 +288,7 @@ static int zisofs_fill_pages(struct inode *inode, int full_page, int pcount,
 	}
 
 	if (poffset && *pages) {
-		memset(page_address(*pages) + poffset, 0,
-		       PAGE_SIZE - poffset);
-		flush_dcache_page(*pages);
+		memzero_page(*pages, poffset, PAGE_SIZE - poffset);
 		SetPageUptodate(*pages);
 	}
 	return 0;
@@ -343,10 +346,8 @@ static int zisofs_read_folio(struct file *file, struct folio *folio)
 	for (i = 0; i < pcount; i++, index++) {
 		if (i != full_page)
 			pages[i] = grab_cache_page_nowait(mapping, index);
-		if (pages[i]) {
+		if (pages[i])
 			ClearPageError(pages[i]);
-			kmap(pages[i]);
-		}
 	}
 
 	err = zisofs_fill_pages(inode, full_page, pcount, pages);
@@ -357,7 +358,6 @@ static int zisofs_read_folio(struct file *file, struct folio *folio)
 			flush_dcache_page(pages[i]);
 			if (i == full_page && err)
 				SetPageError(pages[i]);
-			kunmap(pages[i]);
 			unlock_page(pages[i]);
 			if (i != full_page)
 				put_page(pages[i]);
-- 
2.37.1