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

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Message-ID: <20230627135115.GA452832@sumitra.com>
Date:   Tue, 27 Jun 2023 06:51:15 -0700
From:   Sumitra Sharma <sumitraartsy@...il.com>
To:     Hans de Goede <hdegoede@...hat.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Ira Weiny <ira.weiny@...el.com>, Fabio <fmdefrancesco@...il.com>,
        Deepak R Varma <drv@...lo.com>,
        Sumitra Sharma <sumitraartsy@...il.com>
Subject: [PATCH] fs/vboxsf: Replace kmap() with kmap_local_{page, folio}()

kmap() has been deprecated in favor of the kmap_local_page() due to high
cost, restricted mapping space, the overhead of a global lock for
synchronization, and making the process sleep in the absence of free
slots.

kmap_local_{page, folio}() is faster than kmap() and offers thread-local
and CPU-local mappings, can take pagefaults in a local kmap region and
preserves preemption by saving the mappings of outgoing tasks and
restoring those of the incoming one during a context switch.

The difference between kmap_local_page() and kmap_local_folio() consist
only in the first taking a pointer to a page and the second taking two
arguments, a pointer to a folio and the byte offset within the folio which
identifies the page.

The mappings are kept thread local in the functions 'vboxsf_read_folio',
'vboxsf_writepage', 'vboxsf_write_end' in file.c

Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Sumitra Sharma <sumitraartsy@...il.com>
---
 fs/vboxsf/file.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/vboxsf/file.c b/fs/vboxsf/file.c
index 572aa1c43b37..5190619bc3c5 100644
--- a/fs/vboxsf/file.c
+++ b/fs/vboxsf/file.c
@@ -234,7 +234,7 @@ static int vboxsf_read_folio(struct file *file, struct folio *folio)
 	u8 *buf;
 	int err;
 
-	buf = kmap(page);
+	buf = kmap_local_folio(folio, off);
 
 	err = vboxsf_read(sf_handle->root, sf_handle->handle, off, &nread, buf);
 	if (err == 0) {
@@ -245,7 +245,7 @@ static int vboxsf_read_folio(struct file *file, struct folio *folio)
 		SetPageError(page);
 	}
 
-	kunmap(page);
+	kunmap_local(buf);
 	unlock_page(page);
 	return err;
 }
@@ -286,10 +286,10 @@ static int vboxsf_writepage(struct page *page, struct writeback_control *wbc)
 	if (!sf_handle)
 		return -EBADF;
 
-	buf = kmap(page);
+	buf = kmap_local_page(page);
 	err = vboxsf_write(sf_handle->root, sf_handle->handle,
 			   off, &nwrite, buf);
-	kunmap(page);
+	kunmap_local(buf);
 
 	kref_put(&sf_handle->refcount, vboxsf_handle_release);
 
@@ -320,10 +320,10 @@ static int vboxsf_write_end(struct file *file, struct address_space *mapping,
 	if (!PageUptodate(page) && copied < len)
 		zero_user(page, from + copied, len - copied);
 
-	buf = kmap(page);
+	buf = kmap_local_page(page);
 	err = vboxsf_write(sf_handle->root, sf_handle->handle,
 			   pos, &nwritten, buf + from);
-	kunmap(page);
+	kunmap_local(buf);
 
 	if (err) {
 		nwritten = 0;
-- 
2.25.1