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

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Date:   Wed, 28 Jun 2023 18:15:04 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Sumitra Sharma <sumitraartsy@...il.com>
Cc:     Hans de Goede <hdegoede@...hat.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
        Fabio <fmdefrancesco@...il.com>, Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH] fs/vboxsf: Replace kmap() with kmap_local_{page, folio}()

Here's a more comprehensive read_folio patch.  It's not at all
efficient, but then if we wanted an efficient vboxsf, we'd implement
vboxsf_readahead() and actually do an async call with deferred setting
of the uptodate flag.  I can consult with anyone who wants to do all
this work.

I haven't even compiled this, just trying to show the direction this
should take.

diff --git a/fs/vboxsf/file.c b/fs/vboxsf/file.c
index 2307f8037efc..f1af9a7bd3d8 100644
--- a/fs/vboxsf/file.c
+++ b/fs/vboxsf/file.c
@@ -227,26 +227,31 @@ const struct inode_operations vboxsf_reg_iops = {
 
 static int vboxsf_read_folio(struct file *file, struct folio *folio)
 {
-	struct page *page = &folio->page;
 	struct vboxsf_handle *sf_handle = file->private_data;
-	loff_t off = page_offset(page);
-	u32 nread = PAGE_SIZE;
-	u8 *buf;
+	loff_t pos = folio_pos(folio);
+	size_t offset = 0;
 	int err;
 
-	buf = kmap(page);
+	do {
+		u8 *buf = kmap_local_folio(folio, offset);
+		u32 nread = PAGE_SIZE;
 
-	err = vboxsf_read(sf_handle->root, sf_handle->handle, off, &nread, buf);
-	if (err == 0) {
-		memset(&buf[nread], 0, PAGE_SIZE - nread);
-		flush_dcache_page(page);
-		SetPageUptodate(page);
-	} else {
-		SetPageError(page);
-	}
+		err = vboxsf_read(sf_handle->root, sf_handle->handle, pos,
+				&nread, buf);
+		if (nread < PAGE_SIZE)
+			memset(&buf[nread], 0, PAGE_SIZE - nread);
+		kunmap_local(buf);
+		if (err)
+			break;
+		offset += PAGE_SIZE;
+		pos += PAGE_SIZE;
+	} while (offset < folio_size(folio);
 
-	kunmap(page);
-	unlock_page(page);
+	if (!err) {
+		flush_dcache_folio(folio);
+		folio_mark_uptodate(folio);
+	}
+	folio_unlock(folio);
 	return err;
 }